I only had great experiences with trip.com and ctrip. Every time I enter the chat, the agent is there within a minute and answers my questions precisely, quickly, and without the unnecessary long lines of "You are very valuable to us and we truly understand how frustrating that may be".

The speed did once help me to avoid a 1000EUR mistake when I accidentally booked a non-amendable/non-refundable ticket and inputted the wrong date. While the ticket was being issued (it can take an hour or two when the flight is a month ahead), I texted in the chat and the agent understood me immediately and canceled it. If he would send me the useless text for another minute or two, the ticket may have been issued by then. I don't know if Diamond or whatever status plays a role here.

I used to use Agoda extensively before that and it was a nightmare every time

Apart from China (where it is in most cases cheaper to use ctrip than trip.com), it was also cool to buy UK train tickets there so I did not have to deal with UK apps/websites and enter all the data there again.