You can make criticisms of both. Just actually talk from a place of knowing what you are talking about when you do it.

While I'm not a leftcom, Bordiga is probably one of the more solid critics of Stalin. I disagree with Bordiga's criticism, but I can track and understand where he was coming from because he very clearly actually did the reading.

What I don't always respond kindly to, is someone going "14 kajillion dead, worse than hitler, authoritarian".