A couple days of researching on r/kiwibrowser threads, and mini wars on browser threads on r/privacytoolsio should get you in the loop on arguments used against Kiwi and/or for Bromite.

Not everyone criticising Kiwi is one of the people from the group I mentioned, but a significant portion exists, and they are indeed a significant portion of what makes up arguments overall against Kiwi, even though security patch backporting happens with Kiwi.

There is a bit more to it, in that Kiwi Browser earlier was not open source, and some of the garbage arguments against it carry over from that time, and even conflated at times. It also does not help that these people take advantage of less thorough discussion and solid threads around Kiwi Browser, as not too many people use it yet. Less usage is most likely because of inertia of Chrome/Brave/Samsung Internet mobile users, and Kiwi only offers desktop extension support feature over other Blink mobile browsers.

I was never a Kiwi user until it became open source, and I only use it as secondary browser because I oppose Chromium/Blink monopoly via using Firefox, and also prefer uBlock Origin in all its glory without Manifest V3 crippling.