I feel like on average their conversation quality is higher than the other alternatives. However, as you said, it is not very diverse.
Tildes also feels very “primp & proper” so to speak. While the conversations are generally very civil and of good quality, I also feel like the place is ruled with an iron fist; step a little bit out of line, and you’re pretty much gone. This is then coupled with the fact that you’re not able to make custom communities.
In the end, its good for civil discussion within the bounds of what is already considered acceptable by the admin. I almost feel like it has the opposite problem Reddit does - whereas conversations on Reddit are largely driven by bots and the hivemind, Tildes is controlled by Deimos and whoever Deimos approves of.