You absolutely want to be concerned with this.I would just argue what you're concerning yourself with today may be totally different in that 10-20 year period.

A number years ago, I went to town buying a bunch of rentals in a short period of time and I can assure you they took money from my family instead of putting money in my family pocket.

There is always a transition period where selling owner stays on and over time new owner transitions into full control.

For what it's worth, I went through 5 different property managers in 2 different markets (one local, one out of state) over a period of 5 years, before deciding I would self-manage everything across both geographies beginning last year.

typically with seller finance there would be some period of monthly payments and then a balloon where you pay off the balance in full. 

If you are willing to do this over a long period of time, this can be a great strategy, especially in a low-inventory market.

Although I’ve been investing in real estate, both directly and indirectly for 45+ years, I still get a “queasy” feeling when an investment does not perform as expected.My “aha” moment occurs when we reach the point when, despite the investment problems, we’re able to still show a good/substantial profit at the end of our holding period.  

If something breaks even now, what could it look like in that time period with rising rents, possible refinance if/when rates go down (lenders are offering these with no fees!)

A truly "motivated" seller tends to not really care the amount of earnest money and you could also structure your EM due at end of your inspection period.

Those things can and will hurt your monthly KPIs occasionally and sometimes for long periods.