RE and its numbers just make sense, so if you fail you fail but at least you say you did it..

Amenities got put in, but failed for the most part. 

I wasn't sure if we would fail or not.

As @Tim Schroeder mentioned HVAC but also appliances can be a challenge - hence I when I buy a property if anything is within a few years of failing, it gets ripped out and replaced. 

They fail to make their last two payments, steal your washer and dryer, their dogs have destroyed your carpet and dug up the lawn, there are several broken windows and holes in the wall, and the place is filthy.

It's risky for them and their reputation to put you first in line and have you fail to close a deal.

That will help offset the utility costs if all else fails.

My biggest mistake was failing to account of deferred maintenance in my "down payment".

Quality units in each market will be the high range of that market.I fear if you go below 2.5X, the tenants may not be able to afford the rent and that as soon as something goes amiss that they will fail to be able to make rent. 

If you plan on buying 20 unit buildings, then be sure you have a good manager and a few back ups for when that one fails or plan on managing it yourself.