It's out of that money that otherwise would have had to have been spent on sponsoring super bowls and TV commercials and facebook ads and all that.

This means you really need to trust the sponsor before investing.  

It's easy to qualify the property location and the track record of the sponsor.  

The fact that you also have separate W-2 income should not prevent your self-employment activity from sponsoring a retirement plan.  

It's important to shop around and consider not just the rates and terms but also any additional fees, closing costs, prepayment penalties, etc. as each portfolio loan product is different and they don't follow the standard guidelines put forth by the government-sponsored banks like conventional financing products do.  

You can change managers - When you buy into a syndication, you are are completely dependent on the sponsor.

As @David Lilley said, I can't really see any scenario where I would invest in a deal if the sponsor plan was to refi and then I lost all equity in the deal.

If the sponsor has matured to a level that includes 3rd party administration, they are much more likely to deliver as promised.A properly run fund should never have difficulty delivering a quarterly distribution. 

I’m curious to know how substantial are the returns as the sponsor/GP vs as an LP when the deal is through.

In terms of Syndications -- there are a wide range of syndications and sponsors to choose from, with each deal having a different level of risk/return/sponsor experience AND business plan (development, value-add, etc.)