It’s controversial because the party in power sets the voting rules and they tend to try and do so to their advantage. Be it gerrymandering [1]districts or setting voting rules they think will favor their party. These rules are state level functions so we have 50 different sets.

It creates a system where on political party is pretty much guaranteed to win certain districts and they’re aren’t a ton of competitive races. Part of the problem is an electorate that would “vote for a bag of rocks” if it belonged to the right party.

[1]https://en.m.wikipedia.org/wiki/Gerrymandering