There is just an absolute ton of nuances involved.
SOME types of Federation issues is due not to the local instance but rather Lemmy.World and overall lack of distribution of users and communities across the Fediverse (some of which is better now than the past, but not nearly enough).
Other types involve the instance, and in turn its hardware and even more so its number and skill of admin support. Like if you have to wait several days for a manual sign-up procedure (people say quokk.au was this way, at least sometimes) then you may have already moved on elsewhere.
Some of the issues have greatly improved - like I switched from Kbin.social to Star Trek.Website and for super frustrated with how often I would try to do something - like vote or comment - and so switched to discuss.online, which I have been exceedingly happy with. The thing is, Star Trek.Website's technical issues got WAY better (still not perfect) in the past year, and also I still have had issues with discuss.online - again, most often I would guess that Lemmy.World's lack of updates to the latest Lemmy software was to blame for that (even though I understand that there are a whole bunch of reasons for the delay).
Yet people also report that Lemmy.World itself can be quite slow to access from some parts in the world like Australia and the USA. I don't know how much that has to do with method of access like an app vs. the web UI, and even then, would an alternate front end app like https://photon.lemmy.world/ further affect the speed?
A simple score isn't going to come close to describing any of this. But if it would, uptime % might come the closest? Especially in conjunction with other factors like avoiding recommendation of an instance that has only a single admin.
Discuss.online is tried and true, and I unreservedly recommend it. Anyone who likes can make an alt or two and see tor themselves how good the experience is in comparison between them. Also the admin is quite responsive, both in reacting to requests and remaining on the ball proactively before even being asked - see e.g. the pinned post on that instance.
As the developer himself states, and me as someone who uses it as my primary daily driver concurs, it is not quite ready yet. e.g. a good fraction of the Notifications I receive end up being dead links to posts that don't exist anymore, or to users that I have blocked, etc. Also user tagging is not implemented yet and searching often does not retrieve things that you can find much more easily using Lemmy, plus tools for moderation of remote communities remain very primitive.
Soon now, it will be user-friendly enough to recommend to people, but for now it's primarily for beta testing the software and those of us prepared to use an early adopter mindset when using it - e.g. switch to a Lemmy alt to do things that PieFed cannot yet.
Though more features get added seemingly weekly or at least monthly, it's so exciting to see! I love the new inline comment feature, though inconsistently applied e.g. not yet available for edits. But it's coming!