It's one of the oldest servers, thus if there are server problems, it seems to hit Harbinger 1st. And sometimes, other servers aren't having issues, but Harbinger is down for maintenence. That's not to say that Harbinger is down every week for a night, it's not, but there are times (once every 2 or 3 months) where it seems like Harbinger is down for a night for some kind of extended downtime.
Actually, I have a question about that. I've only recently come back to SWTOR after several months off, and I was wondering if Harbinger stability is as bad as it used to be. Harbinger went down so often last Autumn that I was seriously considering shelling out the cash to transfer all of my characters over to Shadowlands.