From my experience I would not recommend this path for real estate rentals.
I've used these exotic finance vehicles (self directed IRA & ROBS) on two occasions for two different purposes. Neither worked out well.
They can add expensive carrying costs that need to be paid to a 3rd party. You lose some of the best parts of rental real estate, leverage and passive loss deductions.
The rest of the story...
The first adventure with this was buying shares in a privately held MLP. The losses which could have been used against current income, or carried forward to offset future income were essentially lost forever. When the company starts making money, which it is after several years of losses, those profits are subject to tax prior to distribution to the partners.
The second was a start up I funded out of my 401k. The accounting requirements were more restrictive due to IRS special attention than I was accustomed to in my other business ventures. As some have said us Real estate guys/gals don't have the same brain power as portfolio types. If I can't do the books out of my check register or on the back of an envelope I get lost.
Had the business taken off to the extent I thought it would there were some minor tax advantages.
One pro is some have very large retirement balances from which to fund purchases.