Agreed, trams look good, but they aren't able to move as many people as a train because of the limitation of the positioning of the doors. This means that for the same traffic you need more carts, and bigger, more expensive stations.

In cities where the density isn't that high, digging a subway isn't ideal, and you'd probably be better off with a tram, but for high density cities, subways are peak.

Generally speaking, the digging has to be done once, so I think it's a good investment for a lot of cities.