Well, there are better Dark types in the game, such as Tyranitar. There are also better special attackers in the game, like Hatterene. But of course, you need to do quite a bit of leveling up for both, and Tyranitar is only available last game.
SO, IF YOU WANT A DARK TYPE/SPECIAL ATTACKER EARLY IN THE GAME, GO WITH NICKIT
I recommend Nickit for the fact that you can get Thievul early on, even before the first gym due to Nickit spawning on the first few routes and evolving at level 18. I think it looks like a solid in game team member, even though I’ve never used Thievul or Nickit, they have decent stats. The only problem is that Nickit/Thievul both have terrible level up sets, so they rely on TRs and TMs.
So, while there are better dark types and better physical attackers in SWSH, I do actually think Nickit and Thievul are decent enough Pokémon to train up in game, Thievul having good Speed, Special Defense, and Special attack.
Hope this helps! :)