Ten years ago, when most other eateries in Taipei were simple buffets or noodle stands, eating at a buffet like Evergreen was a highlight of a trip to Taiwan for most visiting vegetarians and vegans. And until recently I used to recommend a visit to one such buffet while in Taiwan, with Evergreen being the best.
However, having just been back for the first time in a long time after moving back to Taiwan, I was disappointed to see that they no longer consistently label what contains dairy. Staff are still as helpful as they can be, so if you intend to try one such buffet I still recommend Evergreen over Jen Dow, but they simply had no idea about dairy products. For example they insisted that mayonnaise was vegan, and even offered to show me the packet, and when they did I saw that it was clearly labelled "egg-free mayonnaise" and "lacto-vegetarian". My guess is that with so many better options available fewer vegans go nowadays, and since these restaurants are mostly run by Buddhists who eat dairy products they simply have no idea about dairy ingredients. Pizza (obviously not vegan) wasn't labelled and neither were fake meats or bread products, which are most likely not vegan either.
There are now hardly any vegan desserts, and while they are generally consistently labelled (the only items which are - almost) I tried one item which didn't carry the vegan label but said ("without milk, without egg") but it clearly contained milk powder or or another dairy product.
So as Aya rightly said it's simply not worth the time, cost and hassle of figuring out what's vegan, especially if you don't speak Chinese, with so many better, all-vegan restaurants in Taipei, and more opening up all the time. Go for another hotpot at Guangfu LH or another delicious set meal at Sophie's Garden instead.
Cons: very difficult to know what's vegan, expensive