Let me get this out of the way: Conversion antags are probably my favorite type, because it allows other people to join in on the fun of being an antag. That being said, shadowlings have some issues.
The outcome of the round is determined almost immediately. If the slings make one slipup, then the crew will kill them all. If Security runs into maints right away, slings will win. There is no long struggle over the course of the round, it's always a curbstomp, it just changes who's curbstomping who. Cult also shares this problem, but that's another topic
Shadowstep is too fast. The most common killer of slings is overshooting and reemerging right into space. Also makes jumping from maint network to maint network too easy.
I like the idea of the mindshield not protecting you. What I don't like is how this isn't very intuitive to grasp. Security, especially newsec, often believes themselves immune to mind control, and will rush into maintenance and get thralled, to their surprise, giving the slings a big boost.
The whole hatching thing has caused issues before. Whether it's people seeing the resin or hearing the sounds, a bad hatch can give away the whole thing, due to our lax limitations on antag knowledge.
Let's be honest, how many of the abilities do you actually use during a round?
I'd rather not see them thrown into the trashbin like they are on so many other servers, but some changes have to be made.