"let me count the ways....." the biggest enemy to cash flow is lack of cash flow ( obviously) - but just because a property is positive cashflow right now doesn't mean it will always be. For example the biggest factor in determining cash flow is vacancy - 100% vacancy equals no cash flow. So if your place is not in a desirable area or has too much deferred maintenance, this can be a real cash flow killer if you cannot get it rented out if a tenant leaves. There may be so much cleanup to do that it can eat up a whole years worth of profit. Especially if you have to replace major items like carpet, A/C units, or appliances. The same thing can happen if you have major expenses even while the tenant is still living there - especially if it causes them to stop paying rent and forcing you into an eviction which will eat even more of your profits. The smaller your cash flow is to start with , the bigger the risk of eating into it with major repairs -most investors apply the 50% rule that says your repairs and maintenance will equal 50% or your rental income.
This is just one of the many ways things can go wrong. Another issues is that If your in an ARM ( adjustable rate mortgage) and the interest rate goes up, you can find yourself upside down on cash flow, forcing you to raise rents and possible price yourself out of the market.
Your insurance can go up and increasing your holding costs of the property - and if you made any major improvements , you may end up with a higher tax bill also. All these little things can nickel and dime your cash flow to death.
Personally, this is why I prefer multi-family units vs SFR's because if you have only have one tenant and they stop paying rent or pay late, then you are responsible for the entire mortgage on your own. With multi-family units, you have a little more leeway when it comes to slow payers , as you will have other tenant's rents to help cover the non paying tenant. Doesn't mean you are guaranteed cash flow every month - but it certainly helps cover your expenses in the case of vacancies or non payers.
I'm interested in hearing other investors take on this too.. ....