Almost all ISPs use Serial and (optional) PLOAM to authenticate your ONU. Nokia GPON SFPs allow you to change almost everything that is exposed to the ISP for authentication. ITU standards mean that ISPs have to work according to the spec.
ISP ONUs are locked for easier management for them. They will give you 1/10th of the speed promised if it helps them deal with support calls remotely rather than visiting your place to fix it. Support calls where people complain for things like 'I forgot my wifi password' cost ISPs a lot of money. So they can basically just login to your ONU at any time remotely and change settings for you like your wifi password. They do remote firmware updates and what not remotely. This is the biggest reason why ISPs love such modem router combos. Support can be guaranteed with a phone call to fix your wifi for the average Joe.
You will not get ISP support if you use your own ONU but if you are using your own ONU then you are already at that point where you know what you are doing. As far as signal issues are concerned, like I said ITU specs mean they can see the signal strength remotely. Everything else they don't have access to but you don't need them to have that access. You can always swap out the ISP provided box to troubleshoot.
If you actually swap out the ONU to a better one, chances are you'll never need to call your ISP unless there is a fiber cut or some serious signal loss somewhere.
GPON networks are often vendor locked. This means you cannot just buy any random SFP ONU and expect it work.
Even if you do get it to work, no ISP will offer you any kind of support if you do this. Some might even take to it poorly, if they find out.
Generally, the best option is to ask your ISP if they offer other ONU models and choose from those.