Historians most definitely consider the Soviet Union to have been an empire. Even Walter LaFeber, a "revisionist" historian highly critical of the "American Empire" and its influence on the tensions that led to the Cold War, refers to the rise and fall of the "Soviet Empire," which itself was an extension of the "Russian Empire" that existed prior to the Communist revolution (see his America, Russia, and the Cold War, of which there are many editions).
It is very easy to label the Soviet Union an "empire," as it oversaw a vast territory which included many different ethnicities, some of which did not desire to remain under Soviet influence. Moreover, the Soviet Union exerted a degree of control over its "satellite states" in Eastern and Central Europe. And while the Soviet Union never possessed full control over these nations' internal policy, it did essentially control their militaries through the Warsaw Pact, which always had at its head a Soviet general never someone from one of the other Eastern Bloc nations.
The only real argument against Soviet imperialism would be the anti-imperialist rhetoric of Soviet ideology. On this point, historian Francine Hirsch skillfully narrates how the anti-imperialist ideology of early Revolutionary Russia gave way to the (perceived) necessity of maintaining imperialist Russia's multi-ethnic empire in her Empire of Nations: Ethnographic Knowledge and the Making of the Soviet Union (2005), though I have to warn you that you might find the book drab (though also enlightening). Of course, when we refer to the "Soviet Empire," we usually mean the Postwar Soviet Union which had expanded during the war and which now ruled over the socialist Eastern Bloc. If you are interested in the "imperialist" nature of that era, then I would suggest Zubok's A Failed Empire: The Soviet Union in the Cold War from Stalin to Gorbachev, which is in any case a more entertaining read.
Finally, if you want a case study of the limits of Soviet power over its "empire" in Eastern Europe (which was more limited than most people assume, I think), I would suggest reading Kieran Williams The Prague Spring and Its Aftermath (1997), which does a good job of highlighting the role politicians in Prague played in the Soviet attempt to maintain control over Czechoslovakia (which did not always go as planned, even after the August 1968 invasion). I find the Prague Spring especially interesting because the highly ideological Alexander Dubcek, the Czechoslovak leader at the time, intended to maintain Czechoslovakia's alliance with the Soviet Union, but his liberalization program stirred up anti-Soviet sentiments in Czechoslovakia that Moscow ultimately could not ignore.
This is a difficult question to answer easily for two overlapping reasons.
Firstly, "empire" is a term is incredibly slippery term for historians to define. Empires are often very complex entities and trying to extrapolate some general features means there is a loss of nuance. For example, Charles Tilly came up with a serviceable definition for empire:
An empire is a large composite polity linked to a central power by indirect rule. The central power exercises some military and fiscal control in each major segment of its imperial domain, but tolerates two major elements of indirect rule: (1) retention or establishment of particular, distinct compacts for the government of each segment; and (2) exercise of power through intermediaries who enjoy considerable autonomy within their own domains in return for compliance, tribute, and military collaboration with the center.
Tilly's definition is quite flexible, but also deliberately quite vague and can be stretched too far. For example, one can use Tilly's definition to label the US's immediate postwar position in Western Europe an empire with economic agreements like Bretton Woods, institutions like NATO, and foreign aid like the Marshall Plan as a instruments of an American imperium. This is one problem with labeling entities like the USSR or the US as empires haphazardly, one you stretch these conceptualizations, the term itself becomes distorted and loses its original meaning.
Secondly, applying the term empire to twentieth-century polities often carries critical overtones. This was particularly true for the USSR, where labeling the Marxist-Leninist state an empire carried with it an additional charge of hypocrisy as Moscow styled itself in its public diplomacy as the world's foremost opponents of imperialism. Cold War-era commentators and historians, who were often highly partisan, frequently described the Soviet Union as an empire as part of a broader assault on the legitimacy of the Soviet experiment. For example, Richard Pipes would label the USSR as a "prison of the peoples," and draw connections to crackdowns in Eastern Europe with discriminatory policies against Central Asians and the Ukrainian famine. For Pipes and his ilk, Soviet anti-imperial ideology was merely a stalking horse for Great Russian hegemony. This condemnatory historiography has meant that discussions of empire and the USSR are often freighted with a degree of politics.
In general, historical scholarship on the Soviet Union does not consider the USSR to be an empire in the formal sense of the definitions as those espoused by Tilly. As such, when historians describe the USSR as an empire, it is often in a very qualified sense. Moscow's handling of the multiethnic character of the state it had inherited was decidedly unimperial. The Soviets enacted various policies that often operated against the interests of the Great Russian population. In drawing up the internal borders of the state, the Soviets territorialized ethnicity which created internal republics and other administrative entities to be led by the respective dominant ethnicity (e.g. Ukrainian, Kazakhs, etc.). Inside these new republics, the state facilitated indigenous language instruction, sponsored cultural activities, and, especially in the early years of the USSR, disciplined instances of Great Russian chauvinism. One of the animating policies of the USSR was korenizatsiia (indiginization) which sought to promote native cadres loyal to the Communist Party and the Soviet state. Although the Stalin turn of the 1930s pushed back against korenizatsiia, this idea never really went away throughout the life of the USSR. Entities like the Communist Party, the Komsomol, and the Red Army acted as supranational entities throughout the USSR and were quite prominent social institutions. The planned economy often redistributed industrial and other economic resources in ways that did not benefit the Great Russian core of the RSFSR. Even the Soviet control over postwar Eastern Europe was not as hegemonic as it appeared during the Cold War. Research into the archives of the Warsaw Pact governments has shown that the leaders of the satellite sometimes possessed a greater deal of latitude than prior historiography assumed (one monograph emblematic of this new turn in scholarship in GDR studies has a title that encapsulates this idea: Driving the Soviets Up a Wall).
That being said, although many historians of the Soviet Union today tend to reject the idea that the USSR was an empire, this does not mean that they do not look towards imperial studies to explain the Soviet experience. For example, korenizatsiia often requires a firm definition of what features exactly constitutes a nationality. Both Francine Hirsch and Yuri Slezkine have uncovered how this process often repackaged older tsarist conceptualizations of an exotic non-European other. Soviet modernization drives in Central Asia often fell back upon older stereotypes, colonial discourses and tropes like the veiled and subordinate status of Muslim women. Similarly, a lot of the monographs on the experience of native cadres has fruitfully mined subaltern studies to explain how non-Great Russians navigated this system. Finally, in terms of great power relations, the Soviet desire to cling to its Eastern European satellite meshes well with how classical empires frequently perceived power in terms of a zero-sum game and fought desperately to retain its power.