This will require a credit pull (good credit), required inspections (termite, perhaps wood destroying organism, perhaps wind mitigation), and most importantly a good insurance policy (those inspections must pass, and all safety issues corrected...no holes in walls or floors, siding on, good roof, passable electric system...no Federal Pacific panels).  

Then the rent generated as profit to the LLC is mitigated by depreciation.  

The tenants could sue the LL for prepaid rent and the judge is going to say to the LL "tell me what you did to mitigate your damages"?

Having said that, there's ways to mitigate the risk (e.g., get 3 or 4 quotes - not just one! 

However, if you test and find a high level; then it seems to me that mitigation would be appropriate. 

My opinion is that Radon is the same as any other item that you need to mitigate on a property. 

I don't anticipate 14% maintenance or 10% vacancy, but thinking worst case scenario always helps me evaluate the deal and helps mitigate any unforseen circumstances.

@Steve King Another reason I didn't see mentioned below is also leveraged risk mitigation in an event of an unexpected economic shift. 

If they break the lease by providing 60 days notice, then you are required to market the unit to mitigate your losses, but they are still on the hook for at least 60 days.

The most stringent screening process still can't guarantee you won't run into this again, but certainly can help you mitigate some of those risks (ie. be sure to include a nationwide criminal background check, eviction history, credit score and credit history breakdown, and reference checks from prior landlords).