For the last 25 years I try to book transfers ahead of time, even in the US. And even if I have to pay more.
I LOVE the tour company I use regularly but out of 5 trips I've taken with them, pre-arranged trips have failed for 3 of them. (I always make my own flight reservations and come in a day early so it's not included in the tour cost.)
India- paid extra to the tour company for a taxi to hotel. No-show. I landed there around midnight. Exactly what I was trying to avoid and I wasted time wandering up and down the row of drivers multiple times looking for my name. Booked at an airport kiosk.
E. Europe 1: Arranged with hotel in Albania to send driver (did not pay in advance). No-show but at least it was early evening. Booked at airport kiosk.
South America: asked guide beforehand what service to use, she gave me the name of one that did business from the airport. I showed them the address, dispatcher collected less than I anticipated, driver said it was in a suburb further away and dispatcher had to come out and collect more money(closer to what I'd been told). They didn't speak English, which was fun. Fortunately I knew a little Spanish.
E. Europe 2 with the gypsy driver: I'd just decided it would be easier to find my way rather than waste time hunting for another no-show.
We'll see how it goes when I land in El Salvador next month.
ETA: I see that Viator, owned by the same company as TripAdvisor, offers car service from the airport to my hotel in San Salvador. I can book in advance and pay when I get there. That may be a good option.
And I second Iceland as being very safe. They have ONE prison in a country of 300,000 people. I've been there twice, once as a solo, and hope to go again. On my solo travels I use TripAdvisor or places that put brochures in hotels to get in a few guided tours; it gives me a chance to interact with other humans and learn more about the place I'm visiitng.