Thats not really an issue, its how one connects known to unknown.

The other unknown is losing a quality tenant and potentially get a less than desirable one. 

Upon move out I find that that there is blue paint (about 2"x12" long) on the carpet of the master bedroom and 2 large stains (about 3"x6" each) of unknown origin on the carpet of another bedroom.

They don't have experience, they likely want to move before having an established income record, and there are a lot of unknowns.  

Plus, it is an unknown future expense.  

If you let a great tenant leave because you have a nazi policy that no tenants can transfer to other units - you will lose them, and then have to start from scratch with tenants who are an unknown.  

HOA fees are an unknown, as they can always go up.  

You can Google why, but you won't have control and the costs associated with a HOA are an unknown, and they can change the rules on you in a heartbeat.  

It's an unknown expense - you never know when the dues will go up or there will be special assessments, or if the board (who are just homeowners who probably don't know a thing about managing a large budget or properties) decides to change the rules (including whether or not you can rent your condo out)- or lose all the funds.So, whatever you buy, I recommend it not be a property with a HOA.  

Rent), instead of the unknown issues of homeownership (maintenance..).