I feel like the entire development of Thief was outsourced to a contractor and brought back into the code base without adequate review. From random unintended immunities to instant team kills and other several other glitches, this hero sticks out like a sore thumb amongst all other heroes.
With all that said, the 2 consistent counters to Thief are 1 as stated: Spider eats her and 2: Grand Huntress white kills her half the time with 1 rain of arrows from her white. Thief is really a complete hit or miss in every fight and outcomes are completely unpredictable for the most part. She can do close to 0 damage and die quickly 1 attempt then do 10 million damage wiping out the entire opposing team immediately in the next.
I also think one of the many reasons this character's bugs have not been "fixed" is she is the only character the average Joe can use to stand a chance against a Slayer + Dungeon Man + max starting energy/ movement speed Spirit Wolf team (as she is the only hero that is immune to Spirit Wolf's stun). Maybe when more characters are introduced to combat this team (or less likely Wolf/DM/starting energy are nerfed) Thief will undergo some changes.