Although, the difficulty in finding buildings that will allow (or won't find out) about airbnb'ing can be the most challenging part!  

And then, if you don't refinance it out in the name of the new owner, you face going back into foreclosure again for the due on sale clause that no one thinks the lender actually does, but they actual do, daily.

I completely understand your frustration and am very sorry for the challenges you have encountered. 

When it comes to older office, in particular multistory office buildings, the challenge is often looking beyond 'it pays a decent return' and digging in to understand what is required to day to keep vacancies low, and equally or more importantly what will be required tomorrow to attract or keep tenants?

I can tell you that my most difficult or challenging thing is contractors.  

If yes, that's where the challenges begin.

If you reject a tenant based on any of the protected categories you could face a discrimination law suit. 

An occupied property comes with additional challenges.

Who knew it would be this challenging to find one.

I know that all sounds harsh, but if you aren't up for the challenge of being hard, than you need to hire that out.