My Samsung tablet runs 4.4.2 and since I rooted it, it tells me "this device has been modified and cannot be updated". Sounds like I'm not missing out on much!
This might just be Samsung's "take" on the update. I have an old rooted Nexus 7 (2012), and it updated to Lollipop just fine. Performance is quite poor, but the update process itself was perfectly smooth.
Was the performance on your 2012 Nexus 7 already very poor before L? I had read that the missing trim stuff came in kitkat, but my Nexus 7 2012 is painfully slow.
You are, I have a very old galaxy tab that runs lollipop and it's much faster than previous versions.