Criticism is absolutely fair to the band, but a lot of the time some fans go in with the expectation of being disappointed. Never waiting for the actual release and listening to new music for what it is, and not in comparison to what’s come before. The constant negativity is unnecessary.
The band are allowed to make whatever they want to make, and I'm allowed to listen to whatever I want to listen to. I'm not going to blindly love everything a band does just because I enjoy most of their stuff.
Everything that any artist or a public figure does or release can face criticism, their first albums that people consider masterpieces were criticised by people at the time it's normal you can't shut the fans up.