Insurance companies do a better job of "type rating" pilots than the regulator does. I'm not saying insurance companies are great at it, just better. They may take the time to get to know a pilot, and insure that pilot appropriately. I am insured on a fleet policy for a maintenance and modification company, and can jump into and fly any piston single or multi they work on, up to a certain value. I am legally entitled to fly any of them, but obviously training is vital. Most times I can get training, though there are occasions where it is a read the book and do your best situation.
Certified aircraft are required to conform to a number of norms, including ease of flight, and predictable handling characteristics. That said, it is not reasonable to expect that all aircraft are simple enough for a the jump in and go approach, particularly for a low time pilot. I have flown small certified GA types (albeit odd ones) which took every bit of skill I could muster to fly well. Horrible directional control on the runway, and reversing control forces in several configurations can really catch the unwary badly.
Though I do not advocate more type ratings, I very certainly advocate that pilots not jump and go in some types. Vast piloting experience can certainly mitigate this, but some planes have quirks, and training, or at minimum a briefing, is necessary to assure safety.
A 100 hour Cessna 152/172 pilot should not just "jump and go" in a tail dragger, a float plane, a ski plane, a Piper Tomahawk (though I still think they are good planes), or aircraft like a Bellanca Viking, Lance, or Centurion, which are lots of power on a little plane. Floatplane pilots should not jump and go in flying boats, and vice versa. Twins and turbines are beyond the scope of consideration for my generalization here.
Perhaps the 100 hour pilot is quite skilled, and can pick it up quickly, but some training, and a "signoff" by the training pilot is vital. Happily, I would expect that insurance companies would demand it anyway