Honestly I think it's to be expected that Cavill is not going to shine with amazing acting talent in this film... because he's never really shown himself to be an amazing actor. He's given some good (not great) performances as well as some seriously bad ones IMO.
He wasn't chosen for this role because of oscar nominations. He was chosen because has a good natural 'Superman' energy about him, can wear the suit without it being awkward or 'silly', he is a very committed person when it comes to training (both in building his body up and learning fight sequences), and he has a pretty much spot on look for the role.
And that was a big ask to find a guy with all those qualities. None of us could come up with anyone better in the casting discussions going on back in the day, and the other two guys rumoured to have been up for the role, I very much doubt would be better for the job (Maganiello and Goode).
Not to mention the fact that he is surrounded by a cast that are pretty much ALL well known and recognised brilliant and experianced actors. He's bound to have a more raw energy to his performance compared to them, and I guess to some people that can come off as inexperienced or a bit stilted.
It could go either way for me. I enjoyed his performance in Immortals. It's not 'great' acting, i'll admit that, but there's just something mesmerizing about him and the way he performs. Reminds me of Andrew Garfield actually - some people love the kind of performance he gives and call it talent, others think it's awkward and painful to watch.