Can be a great way to get killed. Your control is non-existent, as is regulatory oversight. So the reputation and ethics of the GP are paramount.Private issue stuff.
I am going to mention one more thing. Many doctors and other professionals get private placement offerings. I have seen many lose their shirts on these deals.
Private placements aren't usually LLPs, though, at least in my experience, they're just unregistered stock that can only be sold to qualified investors. I've invested in a couple of these, and still hold one (a de novo bank). Lack of liquidity can be an issue, thats for sure.