Levels on monsters instead of CR is such a minor F'ing issue. It's not "huge" it's irrelevant. Mostly because CR is irrelevant. You could just as easily use levels on monsters in DnD too, it's just syntax.
The only "issue" is that you see that it say's "level" under the monsters tag, instead of "CR".
If there's one thing people need to stop being so obsessed over it's their damn Monster Manual. Honestly guys, most experienced DMs will at times homebrew monsters, to get them to fit their campaign. Hell I ran a whole dungeon with only homebrewed ghosts and skeletons, in order to get variety at the level the players were. Last night my group fought a Massive Mimic, which was a normal mimic bumped up in size and strength.
And "using your knowledge", sure, but that's metagaming, and the game gives you all the meta data on the monster by examining them.
I really don't have an issue with that. But I can agree with the "lower AC, more HP" in general is an issue as it benefits some classes over others.
