I've been re-watching Kaywhyn's LP of the version 4 of the pack and noting backroutes and other issues. These are just notes so far and have not been tested.
Picnic:
Probably not going to say much as most of these are <do anything you want> levels.
Devious:
11 - Cry for me - make central pillar taller + add one way wall facing left
14 - eight little problems - this level has an embarrassing mistake. Can't believe this wasn't caught this sooner. I've edited it myself just now but I'll have to test later. The upper right most exit should not walk freely to the exit; that should be a splat fall.
17 - Keep it simple - make horizontal piece over gap longer so building from left isn't possible
21 - Delimiter - make gap larger, increase builder by 1
Not much else yet: its time consuming and my time is limited these days sadly.