I succeeded in my (UK, funded) physics doctoral program, but nobody else who started in my department the same year did, even if they seemed to be very capable

I succeeded in my (UK, funded) physics doctoral program, but nobody else who started in my department the same year did, even if they seemed to be very capable and intelligent individuals. I think one or two managed to convert their progress into MPhil degrees. I think the absolute biggest factor was supervisor - mine was new, very supportive, and pushed for my work to be relevant. Theirs seemed to mostly use them as cheap engineering labour, and they seemed to spend the majority of their time polishing optical fiber and binning LEDs. Those that I am still in contact with also managed to get a career in science - this was probably helped a little by the fact they had spent time doing their doctoral program, but I'm sure that while it wasn't a complete waste I don't think the time was entirely necessary either. I wouldn't go quite so far as "strongly discourage", but it's certainly a decision not to take lightly and you should go into with your eyes open. Also, from what I've seen, experimental Physics is pretty low on the abuse scale compared to some of the other sciences, so suspect general experience is _very_ sensitive to the particular field you are interested in.