Where OnePlus excelled over Pixels (at least at one point in time; I've owned both and gone back to Pixel) was that the OnePlus' beefier hardware meant that the camera startup time and autofocus speed was much faster. The cameras were comparable, but pulling out your Pixel and having a noticeable delay between double-tapping your hardware button and actually being able to take a decent picture was painful. OnePlus solved this with their camera software and beefier specs.

Somewhere along the way, the Pixel caught up, and the other quirks of the OnePlus diminished the relative benefit (I recall having some issues with their Android variant, and their charging system not actually following the USB-C spec in a way that was causing me issues). As someone who generally doesn't care about smartphone specs aside from "can it last all day?" and "can I take decent pictures without giving it much thought" the OnePlus line was briefly a great option, but hasn't done anything to make me want to try another one in a decade now.

I can't speak much to other flagship phones; I'll never own an Apple product and my experiences with Samsung's software across other devices means I'll likely never consider them either.