I do know that Firefox has no plans to deprecate webRequests API (that the non-lite version depends on), while also supporting declarativeNetRequest (that the lite version depends on) for compatibility.

What I don't know is:

1) whether their implementation of declarativeNetRequest has that arbitrary limitation

2) whether uBO Lite ships the same (limited) filters in the Firefox release.

I'm guessing 2) is true for simplicity, but that's purely a guess.