Long story short, Dev used to be heavily antiquated with known troon
Lillith Lovett, hosting a podcast with him and another YTer named Scrump. Dev often gives off the impression that Lillith has him on a leash, and he gets a lot of flack for trying to "both sides" a lot of issues, and that includes him bending over for troons, like what happened during the Dylan Mulvaney Bud Light incident in 2023.
That, and Dev's often embarrassed himself in a lot of public places, like Arch's podcast, or the many times he went after Vtuber Kirche. I still like most of Dev's videos, given they're more politically neutural in a world of HBomberGuys, but I can perfectly understand why people hate him.