As I understand the so-called Fishtesting, people write patches, then try the patches against the current version of Stockfish. If the new version performs better, it's promoted to be the main version; otherwise it's rejected.
Question: why test against the current version of Stockfish? It's worked in the past, and current Stockfish is a lot stronger than two-year old Stockfish. However, we can also see limitations to this approach. Although Stockfish is now capable of utterly destroying other traditional engines, it also recently lost the TCEC S15 superfinal against Leela. Therefore, why not test each new version against Leela?
The obvious answer is that Fishtesting's resources come from CPUs, and Leela is awful on CPU. However that shouldn't be fatal: one could just give Leela more time, e.g. with Stockfish running on 10s per game, give Leela 100s per game or whatever time odds are necessary to get the Leela ratio to the ratio in the superfinal. This will certainly slow down Fishtesting, but if it leads to an engine that can beat Leela in a match, that might still be worth it.