Largely, but not everyone has unlimited data (especially not when traveling), and my laptop battery is much larger than my phone's too.
I haven’t really had much signal problem and I’m not travelling through particularly built up areas (on the New York to Miami train, nearing the GA/FL border)
On most trains I've traveled on, Wi-Fi (if available) also works much better since the antenna for that is usually on top of the train, and windows are sometimes coated with a metallic paint to keep out solar radiation.
In the EU there are no roaming costs so it's not any different when travelling.
Besides that, neither tethering (for reasons other than cost, mentioned above) nor international roaming (increased latency) are a perfect replacement for fast local Wi-Fi at this point.
(also: increased latency for roaming?)