They have repeatedly released ROMs full of bugs, crashes, and required factory resets on major version upgrades. Their update lag has increased since being combined with Oppo/ColorOS, while their value has decreased/cost has increased.
They've peaked in terms of hackability and software support.
Even if we disregarded their (worse than google but better than say, Xiaomi/moto) software support, which is more important than the hardware itself, OnePlus hardware is frequently worse (mic, speakers). The only win you get here is a latest gen SoC for pricing that is barely (~10%) cheaper than the Samsung counterpart with a worse build quality.
OnePlus' proprietary warp charge isn't doing then any favors either. Non-standardized charging is frequently a pain point for me when most other devices have moved onto USB-PD.