Autopilot operation is indeed mandatory on some bizjets, above a certain altitude, because it flies too close to mach 1 and can become very hard to control (for a human pilot) in pitch.
Speaking as someone who went from a C150/PA28 straight to a TB20, I had about 10 hrs' diff training and that was plenty for VFR flight. Then I did the IMC Rating in it (no point in flying something like that VFR-only); another 20-30hrs. No problem.
Training how to do flight/weather planning for the sort of missions one is going to want to do is something else altogether. There simply isn't anybody in the UK who knows how to do that; VFR or IFR. To this day I have not met an instructor who has even 1/10 of a clue how to do European touring flight planning combined with this level of (limited) mission capability.
I don't think it is flying a nice 150kt plane that is going to get somebody killed. 150kt v. 100kt just means you have to think 1.5 times faster but that's no big deal; it comes with doing a bit of regular flying anyway (which is a fair point; you can fly a C150 at 6.1 hours a year and stay alive, and many do just that, but you can't fly a decent plane at that level). The "slippery" bit means you have to plan ahead a bit but if you get that wrong you just end up overhead the target airfield, still at 5000ft and doing 149kt, so you end up looking like a d*ck, but it won't do any harm. It also means it can go into a spiral dive faster, but that's a pretty basic sort of inattention, IMHO.
What will get you will be stuff like flying into a mountain, or collecting six inches of ice and plummetting.