My personal experience (oh so many years ago now) with early U-series GE units was that they played havoc with controlling the train when mixed in with other types.
Early U25s had a simple, load regulator type of throttle control, with no feedback loops for horsepower for any one throttle position. This means that as speed reduced under power, even with the throttle reduced to lower notches, load would continue to increase tremendously.
If you had one of these in the lead, you could see it on the ammeter. If it was back in a mixed consist, you had no idea what those units were doing back there as the train speed reduced down to a stop under power (yes, power braking -- we used to do that all the time).
If you weren't careful, you could yank the guts out of the end of a car.
Just another reason I never liked the early Erie, PA attempt at building diesel-electric locomotives.