Don't forget doing STRs to travelling workers. 

The bank made loans to a bunch of amateur speculators who over - leveraged, who didn't know how to calculate cash flow using worst case scenarios (or didn't bother to), and they all failed when the market got tough.

Also knowing the details of the condition of every neighborhood and keeping up with the RE news out of state or even out of your area is tough too.

I have a couple of rentals that I am marketing right now and its tough to find qualified tenants.

This may be tough if the space is super tight for a few reasons, but worth considering.I could talk water heaters all day...

Flipping in Mass is tough just due to the cost of entry.

This is a tough one. 

If you want to manage the project yourself, you'll have to reach out to an iron-worker/welding company.

That co-worker who is obnoxious and rude.

Getting approval from the lien holders is tough enough, but if it's a regular bank they may not make it feasible to assign the contract.