For some it’s a trust issue since they F-droid is a middleman however with reproducible builds I don’t have that worry. Feels like a good thing to have the code verified twice. I have obtainium as well but had issues with some beta apps I wanted to use like organic maps. Obtainium only works if it is the way it’s setup. Organic maps did not follow obtainiums way
Also to use obtainium you actually need to know the app exist, I have a lot of stuff from F-droid I would not have found without it