I hate to say it, but Signal. Signal has consistently been a pain to use for my moderately sized ( Messages sometimes don't arrive or arrive out of ordered and appear in the wrong order, scrolling up has random jumping behavior, opening the chat in iOS causes my audio to stop playing, there is explicitly no way to back up any of the chat, copying multiple messages is broken on desktop, search is super slow and search…
Signal desktop has been broken for almost a year for me "Error handling incoming message" is shown instead of each message. Theres no easy way to transfer messages between devices out-of-band when migrating to a new device (e.g. via encrypted binary backup blob). Messages constantly fail to arrive when they're sent, I often get them days after the person sent them. etc. I could go on...