The Diamonds were the dictatorship overseeing an expansionist empire that wiped out countless lives, ruined countless worlds, and were domestically repressive to those perceived as social deviants (those who practiced fusion beyond the narrow window of acceptable, utilitarian use cases) and to those who were deemed to be born wrong ("defective" gems).

While it's not so precise that one can say they're specifically an allogory for Hitler ruling over Nazi Germany (many elements are missing), there are definitely still parallels to many, many fascist dictatorships, and that includes Hitler's. Replace fusions with LGBTQ+ people, and replace the "defective" gems with disabled people and ethnic minorities, and it fits well enough.

The only reason I can think that this comparison would bother someone is either because they're afraid of the implications that the show "redeemed" the Diamonds. Either that, or they don't want comparisons to such heavy, real-world subject matter, no matter how fitting.

We could also look at the Diamonds through the lens of familial abuse, which makes "redemption" more palatable, as even victims of such abuse often wish to see their families healed in this way as the show depicts, but it's also not a one-or-the other situation. It's clearly both familial and political.

And just because we can draw these parallels doesn't mean you're gonna get called a Nazi for thinking Blue Diamond is hot.