Computer & Video Games has kicked up a fairly typical review of Pirates of the Burning Sea. It seems that reviewers are homing in on the fact that PotBS has a higher-than-average learning curve, hence the somewhat mediocre scores.
Scoring it at 7.2/10.00, the C&VG reviewer said:
I'd be mad to recommend Pirates of the Burning Sea to everyone; it's so fundamentally flawed that most gamers are going to simply bounce right off it. If you're going to try it out, be aware of its faults. I very nearly gave up after just a few hours. I'm very glad I didn't.
It's a fairly comprehensive review so it's worth the read at the link above.