Bellingcat's reporting takes for granted that a BUK was used based on a good amount of evidence there there were BUKs in the area and there was a missile fired from the ground. They didn't and couldn't prove that it was what hit and downed MH17, they didn't have access to the physical evidence and flight recorder.

They seem to have done a lot of good work trying to track the launcher down and analysing the evidence that was available to them, though I haven't been following their reporting. Without the physical evidence being analyzed the link between the BUK and the plane was strong but not confirmed.

Bellingcat was untrustworthy pseudoscience.