Coming into Civ 6, one of the reasons Ed Beach gave for dropping the Civ 5 engine and building a new engine from scratch was they wanted to do things with terraforming that wouldn't be possible with the Civ 5 engine.
At the time, I assumed this related to canals and tunnels, although now we can see it actually relates to ice melting and coastal tiles flooding.
The climate related tile changes seem to work well (setting aside opinions of the climate change mechanism as a whole, just the terrain aspect of it seems to work well). Canal, mountain tunnels and railroads, though, don't make use of the terrain change mechanisms. They feel like they've been slapped on because people asked for them. The tunnel mechanic in particular feels very disconnected with the rest of the transportation and movement mechanics.