Sorry for the late reply, but very often the provider equipment is not great. E.g. with my provider it's kinda random if you get a Genexis or Nokia ONT and people have a lot of issues with Genexis ONTs. I think it is because with the relatively low subscription fees, a more expensive ONT eats out of their fees. In fact, our ISP uses the same FRITZ!Box 5590 that I use for business customers.

There may also be a latency benefit of using a device that is an ONT/modem/router in one. It's one ethernet hop less, but I haven't measured.

tl;dr: when you use your own ONT, you have a choice of picking a known-reliable option.