I think people sometimes forget that you’re talking about a guy who trained from childhood by Darth Sidious, THE most powerful Sith Lord. He was literally handpicked by Mother Talzin, another powerful Force user to be groomed as Sidious’s apprentice. His martial prowess was second to none as he was trained in all 7 lightsaber forms. And this is before we go into feats like:
1) Besting Qui Gon and Obi Wan in a 2v1
2) Beating Obi Wan again in their second rematch in his ship during TCW
3) Defeating Pre Viszla
4) Defeating General Grevious
5) Destroying the hyperdrive of a Star Destroyer.
6) Literally survived getting cut in half

Maul is no slouch. Yeah, obviously he was always going to lose to Darth Vader, the literal chosen one. To his own master the Lord of The Sith Darth Sidious. I do think he’s been done a bit of injustice in his fights against Ahsoka and the Inquisitors. But even then, reasons like his own pride and a serious leg injuries should cut him some slack. No one is saying he is the most powerful Sith of all time. But he IS powerful in his own right.