We have converted most of our fleet of pushback tugs over to TLD towbarless. Push as light as a 757 and as heavy as a 777, they are great. They turn on a dime and will give a warning before they lock up if you are applying too much stress to the gear, when you set up you select the aircraft you plan on pushing. I am a freight dog and push fast, no SLF to complain.
I had one experience approaching an intersection where I felt I may not have been able to stop in time, the only thing I can think I did was accidentally select a lighter aircraft during set up.
As far as the old tug and tow bar, no need to worry. The great majority of shear pin failures are not a result o excessive speed but a result of a tight turn. During a straight tow depending on the tug you can build up some speed. It is up to your operator (esp if he is an engineer/AMT) to use his good judgment on speed taking into account stopping distance and congestion etc.