I've only finished the first three. My main concern for the season was it would get too complicated. I was relieved that the first couple of episodes were easy enough to follow, but by the third I was starting to get a little confused. It was a lot to keep up with three different timelines, it's harder with multiple timelines in two different realities.
I do like the way everything is mirrored - literally - in the alternate timeline. I'm assuming they simply filmed them normally and then flopped them backwards. (The Kahnwald house and the Nielsen house are mirrored images of themselves - the staircases are on the opposite sides, Jonas's bed is on the opposite side, etc.)
Doing a complete rewatch of the first two seasons really paid off. I was able to pick up on all the little details they were repeating in the mirror world, like Hannah finding the hair on Ulrich's hoodie and smelling it, just like Katerina did in the first episode of the season one.
The problem with a show like this is, of course, the younger children are visibly older. There's nothing to say they weren't born earlier in the mirror world, so that's hand-waveable, but they shouldn't look as old as they do in our world. So far they've been careful not to show Mikkel very much but Elizabeth is noticeably more mature.