I don't know since I haven't used them yet. I imagine the middleman is fairly well aligned with you because their business model is to take a cut of the transaction. If they're getting 5% of a $100K or $500K transaction, that's a fairly good incentive not to try to screw people over. Disputes cost time and money.

The party that isn't aligned is the employer. As mentioned, they hold right of first refusal, and they may balk at having "strangers" on the cap table.

I'm sure there is a whole bunch of "interesting" paperwork and if you had a lawyer look at it they would gladly take your money. I believe each company is different, because the way you hold private shares depends on the company governance. SharesPost has presumably done some of the legwork for you because they are advertising this facilitation only for specific companies, which they have hopefully vetted in some way.

I imagine that SharesPost exists precisely because people had to get lawyers and accountants involved to do these custom transactions, and now they are pushing it to the lower end of the market. But they are also fairly new so they can't be perfect.