Approximately the same as would happen if the developer of a traditional Windows* application were hacked or sold out, except less serious, since they wouldn't have access to the underlying filesystem. It's not a non-existent risk, but it is a trade-off I and many other people want to be able to make. uBlock Origin being FOSS and popular considerably mitigates the risks. (Not to mention that I trust gorhill not to sell out...)
* or Linux (except that in Linux most software is distributed via repositories, which lessens the risks, somewhat; OTOH uBlock origin is also available via the repositories, for instance on Debian[0] and Ubuntu[1]). I'm not sure about sandboxing on MacOS.
[0] https://packages.debian.org/stretch/webext-ublock-origin
[1] https://packages.ubuntu.com/search?keywords=webext-ublock-or...