I was able to drive the value through my effort and sell for a profit.

For example, here in Columbus we've got Intel building their mega-fab which is already driving up property values and rents in certain submarkets.

Class D is mostly rentals (another tip to look at owner occupied stats and/or if the neighborhood is identified for revitalization), the tenants are outside smoking, they are talking to neighbors and solicitors, it is a completely different vibe that is pretty noticeable just driving through.

I'll drive you down the streets of Lehigh acres where they build and show anyone the gap between market rents (real asking prices) and their pro formas 

-you really won't 'cash flow' anything meaningful for 10+ years. 

Other risks, the contractor walking off the job, a storm hitting and delaying the project for weeks, someone breaking into the home and stealing stuff (which happened just the other day to a client of mine), a squatter, or drunk driver driving into the house (happened a lot at my old job as a firefighter lol).

Amortized loans do not accrue interest in a meaningfully different way than a HELOC.

You need to understand how to drive the Net Operating Income to produce a good solid return. 

In many cases, whatever weakness got them in that position (lack of focus, lack of drive, failure to take responsibility for their destiny, etc.) will also make it difficult for them to succeed in real estate. 

STR management is 20-25% (I would recommend self managing or getting a VA), since income is higher you could probably get away with 5% for maintenance / cap ex but remember you need to cover utilities and cleaning for these units which drives up costs.