Agree that vacancy is part of the deal.
In my mind your big tradeoff is whether you can effectively and efficiently do the paperword side of the landlording for one unit. You can clearly do the maintenance part, but do you know how to get credit checks and how to spot a good vs bad tenant... it's a lot of overhead on one unit.
I watch Craigslist to see how long our local PM's advertise units, that is a good indication of how long it might take. We have our properties spread across two PM's. One has a wait list and there are hardly any days of unpaid rent. But high turnover. One has very low turnover but is very picky in qualifying applicants and the units can sit empty for months. I knew this going in and strategically chose the properties to send to each PM, you can interview them and others and watch the local market to find the stregths and weaknesses.
WIth a PM you lose a lot of control, but you also have more time to spend on other things, like buying more properties. We self managed for several years, loved it, but loved getting out of it too. The first was the hardest, because we were so naive even with a lot of studying.