I've met only a few programmers IRL that complained about slack, and even then it was for ideological reasons, not user experience reasons. Everyone else, most programmers and all non-programmers I know like the user experience.
I haven't had any real issues with desktop or mobile apps either.
I use Slack for work, and while it was an order of magnitude better than the dumpster-fire of Hipchat (which it replaced for our team), I still think that the interface is kind of "meh".
Custom emojis are fun, inline markdown is useful, but it takes a lot of memory for something that, to me, seems like it should be lightweight. We've had IM since the 80's, after all. There's a part of me that has a visceral reaction to seeing an IM client taking more than 100mb of memory (though to be fair they seem to be getting improving that a lot.
I haven't had any real issues with desktop or mobile apps either.