Okay, so I have not read that particular book of Suny's (it's on my list, I swear), but I can talk about his reliability as a historian more generally.

So part of what's going on here is that "revisionist" means different things in popular use and in academic use. When we use it in everyday speech, it tends to mean that the person is trying to push a biased narrative that downplays or ignores important facts. But when professional historians use it, it has a very specific meaning, which is that a "revisionist" wants to challenge the existing academic consensus on an issue.

So in the context of the Soviet Union, "revisionism" is actually a very legitimate school of thought that challenges a lot of the default assumptions of earlier historians. The consensus in the 1940s and 1950s was that the USSR was a totalitarian state, just like Nazi Germany, and that there was no room for dissent. That's often called the totalitarian school. The revisionists, who started putting this view forward in the 1960s and 1970s, argued against this. So they argue instead that it really doesn't make sense to lump states together as "totalitarian", that there was actually a lot of in-fighting and disunity within this supposedly monolithic dictatorship, that Soviet citizens had a surprising amount of agency in interacting with the state, things like that.

One thing that revisionism is not, is denialism. They are very separate. Denialists like Grover Furr try to argue for absolute nonsense positions which I'm not even going to dignify by listing, but also I honestly can't remember half of what he argues because it's so wrong.

Some older historians who were in the totalitarian school of the '40s and '50s did think that the revisionists were being Soviet apologists, and there was a post-revisionist reaction to them in the '80s and '90s that argued that the state's control of language was indeed pretty comprehensive and could be called totalitarian. However, the revisionist school never engaged in any denialism — they just tried to reframe the discussion around the weaknesses of the Soviet state as a better way to understand it. And from what I've read of Suny, that's what he's trying to do, and probably what he tries to do in his biography of Stalin.

Edit: I forgot to mention, Suny is best-known for taking the ideas of the revisionist school and applying them to questions about the Soviet periphery and especially the western Caucasus. For that work, if for nothing else, I would consider him very trustworthy.

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If you're interested in a more in-depth discussion of Soviet historiography and where Suny fits in, I've always found this comment quite helpful, and the entire thread isn't half-bad either.

I tend to be pretty accepting of revisionist ideas in Soviet history, so I would wholeheartedly recommend Suny. If you, on the other hand, prefer a post-revisionist line or even a totalitarian-school line and find the revisionist school untrustworthy, which is definitely your right, then some alternatives you might like might include:

Stalin: The Court of the Red Tsar, by Simon Sebag-Montefiore: this is a relatively pop-history biography of Stalin, but it's still apparently based on original archival research. I really did not like it, because of how sensationalized and unnecessarily focused on Stalin's personal relationships I felt it was, but oh well.

Stalin: Breaker of Nations, by Robert Conquest: Conquest is pretty representative of the totalitarian school consensus, if you want to see what Suny and his contemporaries in the 1970s and 1980s were responding to. Also a little bit towards the pop-history end, but less exploitative than I felt Sebag-Montefiore's book was, at least.

Stalin: Paradoxes of Power and Stalin: Waiting for Hitler, by Stephen Kotkin: Kotkin is one of the key figures in the post-revisionist school, so he also opposes Suny to some extent (though they, and revisionists and post-revisionists more generally, are on much better terms than the revisionists and the totalitarian school were), and I think he's a lot less tendentious than Conquest and Sebag-Montefiore.

Stalin, by Robert Service: Service kind of steps back from the revisionism/post-revisionism debate and tries to incorporate both the idea that Stalin was a murderous dictator and that he was propped up by an ever-shifting complex of in-fighting subordinates, while also addressing Stalin's personality without making it too much of the focus, and he really succeeds. Graeme Gill called it "the best general biography of Stalin that we have", and Kotkin is probably tied for that since his second book came out, but it's hard to argue against Gill either.

So if you really can't stand Suny, I would recommend Kotkin and Service.