When I last looked it over (maybe a year or so ago) these problems stood out:

• Immature code base.
• Custom onion network that seemed unlikely ever to have enough users to be very effective against attacks.
• Small limit on chat group size. (I think they have raised it from 10 to 100 more recently.)
• Small limit on media attachment size.
• Desktop support appeared to be an Electron app. (I avoid those because they’re incredibly wasteful of resources, and often suffer from Electron’s many bugs.)

Its design showed some neat ideas, but it was not practical for my needs.

Also, I have read more recently that Session removed forward secrecy, which rather undermines its value proposition.