When there's only one person on your island or you're the only person on their island and there are no more people coming, it's definitely easier for the host to use the - button to end the session because it closes their airport at the same time. Me and my friends would do that to send each other back.
Me personally, would not use the - button when visiting others and I would always leave via the airport because I'm afraid that some people might find it rude if I leave using the - button. I'm fine if they use the - button to send me back, but I would try to leave via the airport out of courtesy. I used to do water trades a lot, and I would only use the - button if the host tells me to do so.
Another thing with leaving with the - button is that when there are multiple visitors on that island and you decide to leave via the - button, you reset everyone's position back to the airport and also resets the locations of special NPCs like Celeste. It's very inconvenient for the host to try and trap their NPC again. I've hosted once for Celeste and meteor shower before, and one of the visitors decided to leave quietly via the - button and I ended up spending another 20 minutes to try and spawn Celeste as close to my airport as I can so that I can trap her again. It was a hassle and I had to apologize to everyone else that was in line.
Also when there are multiple visitors, there's a potential that leaving via the - button might mess up the save data. If one of them has a dialogue open like selling turnips or something, leaving via the - button would mess up that transaction. I haven't encountered any issue with it because I only use it when I'm visiting my friends and I would ask them to send me back.