never launched [update: launched the day I posted this] and one was shut down due to unforeseen problems. I have not seen an explicit statement, but it sounds like the Survival skirmish shut down because you could stealth and just stand there while enemies accumulated, unable to find you. I miss all the fun exploits.
But no, the best bug so far is that the healing soldiers do not heal. This is not to say that they just stand there, which does happen at times, but rather that their healing abilities are not actually healing. Instead, they deal negative damage to you. What is the difference? First, your damage mitigation applies to negative damage, so Guardians in heavy armor might get only half the healing. Second, healing buffs do not apply to negative damage, so you get no benefit from traiting your soldier for better healing, your equipment that increases incoming healing, or even the healing soldier’s healing buff.
Pro tip: If you want to farm skirmish marks and don’t need the xp, and who does with all the quests available, lower the skirmish level. You may lose 10% of the marks, but you finish in half the time.