Is lots of half finished clients rather than one finished client an advantage?

Riot is pretty finished. So you have one fully-baked client, and a bunch of others to choose from if you don't like Riot.

Compare that to Slack, which only has one client, and it gets worse seemingly every day (somehow?).

Yes, the ability to create your own client certainly is. Everyone can implement their client as they see fit, as long as they stay compatible with the API. Naturally, you get many clients with different features.

Besides that, there is a reference implementation (Riot).

Maybe if the one "finished" client doesn't work well for some users, e.g. is cumbersome to use with a screen reader, but there's a "half-finished" client that works well for them.