Keep in mind your access to communities on other instances depends on other people on your instance behaving themselves. If an instance defederates with yours because they don't like somebody's comment, you also won't see any posts. Don't join any instances centered around polarizing topics unless you plan on only having discussions within local communities. Also at the moment it's not really possible for a single instance to support much more than 100K users, so Lemmy.ml and lemmy.world don't really want the signups.
sh.itjust.works Main Community
Home of the sh.itjust.works instance.
Honestly for the new user thinking to join just picking something, anything is all that matters.
Each user has very little content on here right now, so if for some reason they don't like the way things are run on the one they chose they can easily choose a different one without losing much. Profile migration (i.e. copy all your stuff to a different lemmy server) is also something coming that I'm looking forward to.
As a sh.itjust.works user you can see almost anything in this network of websites (500+ servers) except lemmygrad and beehaw. Also the server admin here is pretty cool and seems to know what he's doing, so I'm pretty smitten with my choice.
As far as the likelihood of a server going down, that's hard to say as you have no real way of knowing one will be more reliable than another at first glance. TBH I think the smaller servers in the network have the advantage here, their costs to run the server are lower since they have fewer users and less traffic. If they have the creation of new communities turned off, the space required is far less than otherwise. The main detractor with tiny servers is easily discovering whats on other servers. That's a longer explanation why that is, but it's like that and I've gone on too long already.
My main worry with smaller servers is that whoever is running them may lose interest and decide to shut down permanently. It does seem that the pros and cons tip in either direction as a server's population grows.
I get that worry, but I honestly can't say that I think a small server with little interest is more likely to shut down than a server with huge interest (and it's associated hosting bills). For ex. I'm panning on deploying my own lemmy instance soon, it's going to have signups turned off, but if I left them on I wouldn't really care how popular my server is. The less the better imo because it becomes cheap enough for me to forget I'm admining it until update time comes around and fewer interpersonal issues to deal with because fewer users. Maybe I'm in the minority here, but becoming a huge server with a lot of users doesn't appeal to me like at all, and having a small user base is only upsides. Once I deploy it's going to be running for a long time.
Edit: I should have asked if you meant user interest or low admin interest
What good does hosting your own instance do if it's closed to signups?
So I can access the fediverse with my own domain and own my user profile. Currently my profile is owned by the sh.itjust.works server; which is fine, the admin knows what he's doing, but if it were to ever go down, my user profile and ability to interact on the fediverse goes with it. I'm an avid selfhoster and enjoy having control over my own infrastructure, so hosting my own lemmy instance is right up my alley. I'm not interested in the headaches that come with hosting for anyone else though, as there are quite a few.