They would have been much better had they been the main antagonists of the games. The "they don't want to take over the world with the power of a legendary" doesn't really mean much when they're still just lackeys for Aether. Guzma had a lot of potential to be interesting and add to the story with his grudge against the Trial system and their captains, but because the narrative focused on Lillie's desire to forgive her mother (in SM) or stopping Necrozma (in USUM), he never got the chance to do anything with his hatred. Even simple things like making Team Skull sabotage some Trials would have worked wonders in numerous ways: firstly, it would have made certain Trials actually enjoyable (remember SM's Electric "Trial?" Or what about the "walk in a straight line" Dragon "Trial?"). Second, it could have given us more opportunities to learn about the members. Why is Plumeria a part of Team Skull? What happened in her life that she threw it all away to become a thug? What about Gladion? He's only a "Team Skull Enforcer" for one battle. Having him lead an attack against a Trial could have given us great insight into his character--showing that he's not happy being a part of Team Skull, but he has to do it for the money.* Third, it would have added a more "cultural" theme to the games, which is appropriate given Alola's lore. Guzma would be trying to destroy part of Alola's culture (the Trial system) while Kukui, who has connections with Guzma from his youth, would be trying to uphold it while also adding to it for the sake of the future (the League).
*I never understood why Gladion joined Team Skull for the money. Po Town makes it really clear they're all incredibly poor. I think he would have made more money working at the Thrifty Mega Mart.
