An update on my experiences:
My investments through online real estate have been through RealtyShares, PatchofLand, Fundrise, RealtyMogul, and PeerStreet.
The vast majority of my investments have been in first lien loans. I haven't had any real negative experiences other than some loans being paid late (but they then were charged penalty interest). I have one loan on a property that has defaulted and is now working its way through the court system. I am not too concerned about it, I will get my principal back, plus some of the interest. The borrower has a few hundred thousand of his own cash in the deal which he will forfeit if foreclosure happens. That equity should cover attorneys/court costs/marketing costs of the property.
I have not added to these investments in several months. I am pulling my money out when loans are paid off. I don't want to be a lender when it seems like we are close to a market top in real estate.