I think most if not all characters recover from their get up in place or roll behind you way before you can fsmash, if they buffer a roll behind you and you fsmash immediately they should still be able to shield I'm pretty sureget lucky with down throw
I actually use fsmash a decent amount because it is game changing if it lands but there's no way to land it aside from prediction and certain characters recoveries really, nades and mortars can make it harder to punish if you throw it out every now and then but it's still one of those things you can't really hit with much