Speed was a big thing. Switching channels could take a few seconds to over a minute to load, on good hardware.
The biggest issue, and a huge glaring oversight imo, is that users can create their own channels, encrypt them, and instance owners have no way to know what goes on in there. Some of the channel names alone were enough to make your skin crawl.
Oh, and you want to ban somebody? Cool, just ban them individually from every channel, because there is no global instance-wide ban. Moderation is horrendous.
Hosted my own, was basically trying to move a large group off telegram and onto my own servers.
I did use Mjolnir for a while, but that was a hassle in and of itself as well. Nothing was intuitive.