As someone who has Jack and KH and still uses Jack because I find the idea of a little girl murdering monsters/bosses hilarous I honestly fail to see where the problem lies with her vs the current best Buster option avalaible, she'll take what 1 additional turn to kill stuff sometimes? She's a Quick servant but she does just fine even before Skadi.
I use Shuten for fights that would usually fit a ST assassin better, she's an AoE assassin that will do a lot less damage, she'll still finish the fight, granted not as fast as a ST assassin, but does that really matter outside of farming?
As I said previously I keep using Kiara for any fight that allows me without any trouble and people keep saying she's weak, but I keep having no issues.
Using non-meta servants is fine, you won't be squeeling at big numbers and it'll take a bit longer, but does that matter?
I have used Mordred for most of the game I never had any troubles honestly (and this before she spooked me up to NP5), I don't user her for farming but I still bring her out for bosses because I like using her over Musashi.
I don't particularly care for Cleo but if you like her she's fine, I have used her off FL support during the year and she always did well too.
It doesn't matter if you want to use Shuten over Cleo, or Jack over KH or hell Cleo/Shuten over ST assassin (like I do), your servants will pull through granted RNG won't screw you, but RNG can screw you even if you have the best setup in my experience.
Ofc if you like to play keeping performance as the most important thing that is fine as well.
