This place is across the street from my home and is a nice option to have, but I prefer to go farther afield to places where "vegan" is better understood (Yak & Yeti or better, to all-vegan True Bistro, both in East Somerville). While it's true that Tom Yum Koong has many tofu and vegetable options, none are labeled vegetarian and they don't seem to understand the word "vegan" so when eating there, I've felt nervous that they won't take the care I would like in ensuring my food is not prepared in the same pan as their many fish/shrimp-based dishes nor that their sauces are completely vegan.
I have nothing against English-as-Second-Language folks, but when they don't know the term "vegan", then it can be tough to really know if what they're telling you is "vegetarian" is actually vegan or not (especially since so many people consider fish as "vegetarian"). I have of course asked about fish in the dishes I've tried (Masaman curry, Panang curry, Tofu triangles and others) and they've assured me there is no fish sauce, but it's hard to know if other things (e.g., shrimp powder or honey) are in mysterious sauces and I'm not motivated to talk through a long list of non-vegan potential ingredients.
For what it's worth, their food is tasty; I'm just not rating highly because I don't consider them very vegan-friendly.
Cons: don't understand "vegan"