Decent enough to reach about 90 hit with aim.
All three of those are niche units, though. Sherlock is the bosskiller, Ruby is needed for the best unit in the game and Christine has horse-lover, crossbows a good prf and aim. All three are mediocre at best, but are certainly useful once trained. Sherlock is easily the most explosive unit in the game (barring Brymranger!Sherpa) and is generally considered to be very, very good after the earlygame, and Christine's ability to heal horses combined with so-so combat makes her okay as well. Almost every unit has some situation where they will excel, not just in Berwick, but FE in general. It'd be hard to find units who literally have no purpose.