I lost money (opportunity loss) but I gained a wealth of experience. 2.)

If I had to pay my cleaners on top of the loss income it would impact even more as my cleaning fees range from $100 to $400 depending on the property.  

Most people are happy just to not take a loss each month. 

There are several things to fear money loss on a bad investment, government involvement in RE, bad tenants that cost time and money, hidden maintenance issues and/or land issues and the list can go on.Today, I don't fear money loss or tenants,  the big unknown is how much the government wants to get involved in either taxes or landlord/tenant issues.

It will be a lot of hard work for little to no pay (or loss) until you hit a point that will be long and hard. 

Can you take the loss without completely ruining your finances?

As long as I understand the risk and mitigated the risk with sound exit strategies, I can let people live in my properties and I haven't had any losses yet.

If you are on cash accounting, yes you could do that.On the other hand, you are ignoring carry-forward losses. 

A reserve is unnecessary, but I still keep around $15,000 - $20,000 in my account.The point is, that you should sit down and assess your finances to determine what the worst-case scenario may look like, how much you would need to cover it without impacting your life, and whether you will need to build a reserve.

Grocers must eat the loss of their fresh vegetable or frozen foods during a power outage.