It's the same risk as approaching people for rp. Some people aren't here for that and might find the contact strange or like they don't know how to respond because you want something different from everyone else. And it depends on your approach. I'm not a chatter box about nothing and I've gotten weirded out by folks on rare occasion who just randomly come into my inbox, "Hey, how was your day?" Excuse me, I don't know you. Besides that, if you knew me better, you'd know, I'm not someone who craves talking to people about nothing. I like genuine topics. A random PM about spiders or books from someone I've never talked to before would be slightly odd but I'd be totally game to chat without preamble or need for introductions. Just jump right into art or bees and wasps or movies. Ice breakers that you'd give to someone you just happen to be standing next to in line at the grocery store make me aggravated because we're not waiting in line, I'm actually busy doing something and you're interrupting me to talk about the weather or what I ate today. And who even are you? You know?
But some people are really sociable and are genuinely sitting there looking at the forum title page bored out of their minds. A random PM being casually chatty would be plenty welcome.
If you're someone who is really bothered by ghosting or getting rude declines, then be prepared. Because a big bulk of people who get a message like that will be waiting for you to get to the point or will be weirded out that you don't actually have one. You'd be better served making initial contact in the chatbox. That way you've set a baseline for casual communication. After chatting in the chatbox, people might be much more open to receiving a casual PM from someone that they've seen and talked to in there.