Rewatching early Simpsons episodes as someone who first saw Flanders post Flanderization: He’s a less compelling satire because it’s so nuanced, complex, and narrow.
He’s not the obvious bad person that Marcy D’Arcy is, but he’s also not the aspirational zen master that Wilson from Home Improvement is either. He’s just kind of a normal-ish OK guy who’s not a compelling foil to Homer.
Take his funniest characteristic (calling reverend Lovejoy at night) and make him a broad vehicle to satirize American Protestantism, and he’s actually a compelling character.
On the other hand, Lisa’s evolution kind of sucks.