Just listened to Paul Sztorx's talk on CUSF at the opnext.dev conference.

It is the worst technical concept I've ever seen presented at a Bitcoin conference. 😂

tl;dr: because no-one wants to merge his terrible soft fork proposals – and he's too lazy to rebase his code – Sztorc wants to ship a separate piece of software that uses the invalidateblock RPC call to detect and reporg out blocks that violate his soft forks. Obviously, if miners actually run this, it makes it a nightmare to actually run a real validating node as now this Rube Goldberg bit of complexity now becomes the Bitcoin protocol. And obviously, that makes it likely that a soft fork won't be enforced by economic nodes, resulting in massive losses when miners get bored and stop running it.