I noticed this in the jet charter business as well. I think it might not necessarily the training itself as much as it is the lack of previous experience to build upon. Historically, pilots attained a significantly higher level of skill and experience in light aircraft before getting hired onto a jet than they often do now. I submit that having significant PIC experience prior to moving into jet aircraft makes the skills and understanding one already has more transferable by simple virtue of the earned confidence one should rightfully have in their own abilities. I saw this time and again when I was flying charter for a company that favored entry level (read low paid and even a couple of PTF) FOs.
At another charter company prior to that I was spoiled beyond belief in comparison. They gave me mostly Navy pilots to train and check. No problems with visuals from them. They didn't scare easy and they knew the difference between merely challenging and outright dangerous already. Mostly, all they needed help with was adapting to our procedures. Hell, I learned plenty from some of them. Most of Navy guys got their 1,000 hours or so in the charter biz and moved on to better jobs.
Pretty stark difference between the two scenarios eh? Well, there were also guys like me who came from GA, had some PIC time and a little teaching experience but no jet time. When opportunity knocked... I guess our learning curve was somewhere between the above extremes. I didn't find my own initial transition to a jet easy by any means, but was able to transfer previous learning after some coaching, some experience and earning the self confidence to trust myself like I had previously in piston airplanes.
Anyway that's what I see from my vantage point.