Due to its (afaik) unrivaled privacy-preserving architecture, the Signal servers have systematically less info on you than, e.g. a Matrix server. The risks of an attack should therefore be much lower.

Oversimplifying a little bit here, but I believe the most likely outcome of a Signal hack would be downtime, rather than a data leak. Contrast that with e.g. matrix.org (where the majority of Matrix users are).