[chaircher]

It depends on a lot of different factors.

Locally and even regionally buses go routes trains aren't going but they get caught in traffic and are a lot slower than trains even if it's a clear run. Buses also tend to be less pleasant but cheaper. You can always force more people into a crowded train but buses get to full capacity and have to drive past people waiting for them.

Northern Ireland has a pretty effective coach system that mirrors what would be expected of a train system, but you don't usually get that sort of thing elsewhere. But then Northern Ireland (and the north of Ireland) has some insane gaps without train coverage. https://maps-ireland-ie.com/ireland-train-map

In England getting a coach is basically often what you do because you can't afford a train so they often run maybe once a day, or maybe a few times a day on some of the bigger routes. They often opperate at really odd hours and for travel to neighbouring countries coaches are better than train because they coordinate the ferry for you.