The most giveaway thing is the name itself. When Benoit goes up to talk to miles he sees the illusionary glass onion in his office and he remarks about it. The glass onion means that the killer was in plain sight the whole time Benoit just had to peel the layers of an already transparent onion.

The issue with glass onion is there is no subversion of the mystery. It's pretty much the guy you expect 30 mins into the movie is THE GUY. There's no ultimate like "ha! Got you!" Moment by a super secretive killer.

In knives out, you do not know who it is until it is literally told to you or you pick up on shit super early into the film. There's also like a subplot of saving those people in the laundromat and the firebombing that makes the mystery seem much much more grand than it really is.

I think honestly, COVID really fucked with glass onion and I will give it a pass. Hopefully the next movie is better.