I've been looking into RJ Rummel recently, and, while I respect his project of attempting to estimate democide counts on a per government basis in the 20th century, much of his work seems sloppy. For one thing, his definition of "democide" encapsulates a number of distinct phenomena - political violence perpetrated by the state, political violence perpetrated by non-state actors with or without state backing, famine, deaths from land reform, etc.
To Rummel's credit, he generally does disaggregate between these when producing his mortality estimates, but that brings me to another point: Rummel has sought to comment on so many different periods of state violence that I doubt he has expertise on more than a few of them. He tends to accept mortality estimates from peer-reviewed papers, but it seems like he often glosses over some of the nuances and debates surrounding those mortality estimates.
In general, what do historians think of Rummel?
EDIT: I found at least one text which is critical of Rummel's methodology here: https://sci-hub.st/10.1177/0022343304040051
EDIT 2: The more research I do on Rummel, the less credible he seems. I'm not going to discredit the whole of his scholarship, but he was definitely writing from a certain slant; he accused Ted Kennedy of being responsible for Khmer Rouge killings in Cambodia from his anti-war activism, for instance: "The post-war blood of millions is on Kennedy’s hands". He proceeds to cite his own figures to prove this point, so it seems like there was probably a politically motivated nature to some of his work. If he used a more rigid and precise methodology, I would be willing to overlook this, but, as the article linked above points out, his conclusion for democide in Yugoslavia is contradicted by his own data. It's clear that the process of selecting a "probable estimate" ultimately came down to his personal whims a lot more than any sort of rigorous methodology, and, given that he's demonstrated some pretty clear political biases, I do find it hard to take this guy seriously. His figures for Soviet deaths definitely seem inflated, and, even though I'm not very familiar with Chinese history, some of his statements about Chiang Kai Shek seem exaggerated at the very least.
EDIT 3: I mentioned political bias earlier, but I've found a few more things that also detract from his credibility. Rummel has claimed that Woodrow Wilson was a fascist dictator and accused Barack Obama of attempting to establish a one party state: http://rudyrummel.blogspot.com/2009/07/authoritarianism-on-way.html. So, yeah, altogether, it seems like this guy is pretty openly biased, isn't taken seriously by most real scholars, and has explicitly correlated his academic work and political views.
FINAL EDIT: I've basically answered my own question at this point, but, in case anyone comes to this thread looking for an answer, it turns out that u/Sergey_Romanov has already given a pretty cogent one: https://www.reddit.com/r/AskHistorians/comments/cuoi3a/are_rudolph_rummels_works_about_genocides_in/exx74xp?utm_source=share&utm_medium=web2x&context=3
Focussing only on the problems with Rummel's data misses the value of Rummel's data (I will only consider his compilation of data, and not Rummel's writings and conclusions from the data, so his political views are not a major issue.)
In history as in science, compilations of data can be very useful. Where totals are likely to be important (e.g., if somebody wants to calculate the total number of people who died due to the actions of a particular regime), comprehensive coverage is important. For example, if one wanted to calculate the total number of deaths "caused" by Stalin, executions and Gulag deaths are not enough - at minimum, estimates are needed for transit deaths during forced relocation of ethnic groups, deaths due to disease and starvation shortly after arrival, deaths of POWs, etc. Comprehensive (and therefore maximal) totals were Rummel's goal, and his data is correspondingly comprehensive.
Of course, the data in a compilation of data is limited by the quality of the sources of the data. Where that original data was exaggerated, or invented from thin air, for purposes of propaganda, it will be inaccurate. Where the accuracy was limited by official secrecy, it can easily be inaccurate. The criticism by Dulić (2004, in your first EDIT) is quite appropriate. The criticism of the Soviet estimates because they predate access to Soviet/former-Soviet archives is quite appropriate (Rummel's newest sources for his Soviet estimates are from the 1980s). As you commented, Rummel simply took a bunch of estimates, and usually assumed that a mid-range value was reliable enough. If the estimates are all systematically biased the same way, the mid-range value is biased.
There is a lot of room for revision of Rummel's numbers, where newer sources are available. Rummel estimates about 750,000 deaths due to Soviet internal deportation of Poles; modern estimates tend to be about 300-350,000. Gulag deaths have been revised downward (but the Gulag camp totals needs care: only a minority of detainees were in the camps proper). At first glance, Rummel's famine estimates for the Soviet Union are in the range of modern estimates, and lower than some. For some discussion of the estimates used by Rummel (e.g., Conquest (1986)), see
Going beyond the data itself, Rummel's totals need to be treated with care. For example, he includes 1946 famine deaths in the Soviet democide total, when these were due to wartime devastation, perhaps better attributed to the Germans than the Soviet government. Should deaths due to the 1937 breaching of the Yellow River dykes (mostly resulting from disease and famine) by the Nationalists early in the Second Sino-Japanese War be counted in the Nationalist total or the Japanese total (or neither)?
In summary, Rummel's compilation of data is valuable, but needs to be used with some caution:
Newer and more reliable estimates are available in some cases.
Rummel's data is very broad, and his totals include famine deaths and other deaths that one might wish to exclude from totals.
I have found, for example, Rummel's data for deaths during the Second Sino-Japanese War and the Pacific War to be useful, since estimates are given for the important Japanese-occupied regions. In this case, the estimates are much less affected by Cold War secrecy and Cold War propaganda than his Soviet data. It is, of course, possible to look at each individual estimate to evaluate the quality (e.g., Rummel assumes a low-end total for deaths in the late-WWII famine in northern Vietnam).