The view from New York
After the dot-com bust I spent 6 years working as a train operator for New York City Transit--the subway system. We have both one person and two person operation here, and I've worked both.
There are two problems with one person operation as I see it. First, it makes for slower passenger loading at stations. Our conductors (guards) work from the middle of the train and have a much better view of the platform and can better control door closures to make sure folks get completely on and off the train without blocking a door and delaying the train. The second problem is that it creates a distraction for the operator by having to switch from conductor mode to train driver mode at each station.This adds additional dwell time at each station.
These problems are particular to our implementation of one person operation. I often visit London and think they have done a much better job of it. In particular, London provides plenty of passenger control people on the platforms to help with loading. We have hardly any here.
We have one line in CBTC operation here. It is one of our shortest lines, with little interaction with other lines. The project ran way over budget and schedule and is far from trouble-free. Station overruns are common, as are mysterious system glitches. I often wonder if the enormous investment could have been better spent on other pressing needs.