The "rewards" of being a Jedi are supposed to be greater but take longer to attain. Dooku probably was close to achieving enlightenment or whatever but decided to join the dark side as a "quick and easy" path to more power. So he did grow more powerful, but he never became as powerful as he could've been had he stayed.
They all have fan followings and looked badass as Sith Lords, but the reality was pretty shit for them, lol.
Vader was the only one who decided to stop being a tool and overthrew Sidious. So he reclaimed some badassery in the end 👆
Tyranus died realizing that he had spent his last thirteen years as a tool.
Maul's just a failure, through and through. He was Palpatine's tool up to TPM. He was too scared to come back to his master after TPM and spent over a decade languishing on Lotho Minor as some crazy spider-hermit. He then gets restored to full health and begins to wage a shadow war against... who? He has all these half-baked attempts for vengeance against Obi-Wan that fail, and his little insurrection on Mandalore is swiftly ended by Sidious. He gets captured and manages to break free, then captures Dooku & Grievous (his crowning achievement) and tries to get Tyranus to join him. What ends up happening is that Dooku and Grievous escape and Sidious kills both Savage and Talzin, the only people Maul ever cared for. Cue another 10+ years of languishing in exile.
Now he tries to train Ezra to be his tool, but loses to a blind Kanan. And now it looks like Ezra's kicking his ass too. I truly pity Maul. Dooku may be a tool and a proton torpedo, but Maul was a genuine loss... of Sith dignity and badassery. 🙂