If you can get Diamond, EVA treats their Diamond really well. So if your flying EVA 12 times a year, id say, its worth it. EVA J has been pretty difficult to get though but i thought their redemption is better than many. from LAX--TPE route, its like 50k(Y), 55k(PE), 75k(J) on one way. Whats cool about EVA miles is that the airline will let you waitlist flights that are full and release close in. i dont think united would allow that.
Of course, if you fly more domestic, then united miles earning is probably better. I do believe their mile redemption is much for for say a TPE--SFO route (110k?) or something like that. *A gold on EVA isnt seen highly regarded but if you make a request, they will try to fill that request if its within reason.
As for call centers, i never called united so i can speak for that but EVA call center is a disaster haha. Need to call overseas offices and at the right times too.