People did, and do, get chopped from OCUs but I think that the rate is far lower than for earlier stages of flying training. After all you should only get there if the prevous course thought you would pass!
In my experience of going from hawk to F3 in the mid 90s, I found the hawk to be very poor prep - basic ACT was taught poorly and there were no systems in a hawk - it was still very much based around flying around at LL doing visual nav with a few other bits tacked on. The F3 was far more about kit and SA in a vastly bigger bubble than the hawk, as well as the 2-seat piece. (Also no moveable wings in a hawk - the bane of many a baby Tornado pilot!!)
In terms of training quality I think that we have moved on in leaps and bounds, on both the hawk and OCUs, since the mid 90s. At that time I would rate Valley as poor with far too mny old school types and the F3 OCU as OK. Some of the stories from my contemporaries on other FJ fleets are horrendous. Would be interested to know what rotary and ME training was like back then?