The biggest things I look for are previous evictions and debt to income, do they make enough to pay their rent?

Biggest issue you've identified - Investor A has to own real estate to sell and 1031. 

The biggest challenge has been being oversees for a lot.

Yet they don't follow up.So -- biggest advice I can give:  Don't be like those guys.   

But, the states have every intention of going after the biggest offenders and making giant examples of them.

The biggest challenge with BRRRR is Refinance aspect.

Right now my biggest fear is how much to leverage as all CORF seem to require personal guarantors. 

@Jeff Faulkner yes and no. of course it’s ideal to pull all of your invested money back out. of course that’s the ideal scenario but i don’t think it defines the strategy. to be honest i’ve done 2 brrrr’s this year and have done very well on them but i did not pull all my money back out. if you really get down to it, there was money that i spent above and beyond what the hard money lender reimbursed and also, i didn’t get back all of the interest at 9.99% that i was paying to the HML.i got a check for 19k at closing. i expected it to be more but am still happy. don’t forget to account for closing costs out of what you get back as that was a 10k hit for me both times. biggest issue for me with brrrr that is actually somewhat out of your control is the dang appraisals. all this talk about rehabbing a property and having the appraisal come in crazy high, yea i haven’t seen that happen.

I've talked to many investors whose biggest frustration was purchasing what appeared to be a great deal but having limited professionals to manage and maintenance the property.

As far as it being the biggest I'm not so sure.