On the handful of times we have found someone in a unit we have threated to call the police and it has generally worked, they would need to show something that ties them to the unit (mail, utility bill, or lease) for the police to not get involved.  

But the threat sounds good either way. :-)

Then legal threats were made.  

For example declaring a monetary war thus necessity, or a cyber war etc etc.. 

My insurance company won’t let me rent my 2 car garage or an office space that I have because it poses a “threat” of a non-tenant person coming onto the property and potentially posing harm to my residential tenants.

As you will deal with every horrible thing in life.... tenants disputes, sleazy contractors, the city who hates your grass, everyone has a sob story and can't pay rent, be emotional ready for evictions and threats, and know your lease and local laws.

@Albert Johnson  I am sure there was likely a threat to the first plumber. 

Wildfire threat (especially for RE investment) is potentially catastrophic. 

There have been a several threats of a lawsuit.  

Things that are within my skillset include: electrical (unlicensed), IT networking, cyber defense, and any handyman tasks.