For me the biggest worry with sending PMs was always that you may get no reply at all, nada. Sure, theoretically you could get a very rude reply (which is even worse, at least for me), but it happened to me just a few times in my whole life. “No reply” is a much more common situation (though still rather unlikely in ALL, I think.) No reply could mean just anything. For example:
Type 1 situation: They don't see much in common with you, or don't like something about you, they're just trying to be considerate and they don't want to hurt your feelings (nor want to lie to make an excuse). So, they don't reply at least partly for your own sake. Which is usually not a good idea, because you may be afraid that it could be a "Type 2 situation" - see below.
Type 2 situation: They think you're a creep, a stalker, a very unpleasant person (for them). So, they don't reply to protect themselves. They don't want to touch you with a 10 foot pole.
These are just two extreme examples, there're countless other possible nuances.
I've made a rule for myself: When I send a PM, I don't expect any reply. Or at least I imagine the situation that I don't get any reply. Should the foreseeable effect be devastating, maybe it's better not to PM in this case. Sure, even if you try not to expect a reply, and you get none, it still hurts deep down, but not so much. And if you get a reply, it's like a small X-Mas present.
Sure, for a lonely person it's much more difficult. Someone, who already has many friends, will just shrug it off: “So what if they don't like me or even find me <this> or <that>? Perhaps, I'm better off without talking to them (anymore).” On the other hand, if you hope to make your first friend in a long time (or ever), you bet a lot on it and you measure yourself by what the other person sees in you. If you realize this, you can prepare yourself, but still it will never be as easy as for a Mr./Ms. Sunshine.