I still don't think it' a very fun game mechanic. If you RP it properly, you die pretty quickly, which sucks. Generally that means that you become an alien, which (I think) is boring, since it entails practically zero RP.
It'd be better if more symptoms started showing up, so you had time to a) notice the symptoms b) Ask a doctor for a checkup, c) For the doctor to discover the foreign body, and d) For the doctor to remove the alien. Then, as long as you don't tough out whatever illness you may have and you also have a capable doctor, you won't die. Suddenly, aliens have a reason to worry about the medical clinic, because it can prevent their reproductive cycle. Not having access to the medbay suddenly becomes a tense issue, and if you know what's going on you'd still have a chance to break some bottles to attempt adhoc surgery.
Aliens are also incredibly boring to play if you get most of your enjoyment from roleplaying. Once you're an alien, SS13 turns into a 'tackle the humans' rogue like, with any discussion being limited to meta-jokes and tactical discussion.
So, that's my beef with aliens game mode. It's a little fun for the humans until you actually have to fight the things, in which case there's no dramatic tension being you're almost guaranteed to die, and it's a little fun for the aliens if all you want to do is hide and kill people.