Information about power settings is published in the Flight Manual, as are any operating limitations, which will also be marked. If there are any changes to the operation of the aircraft resulting from avionics installations, there will be an FMS for these. Otherwise, the use and operation of this secondary equipment should be considered optional, and will not affect the aircraft if improperly done.
Certification of the aircraft, and the equipment installation, would not allow approval, if the aircraft is very difficult to fly in any configuration. The pilot is required to be familiar with the contents of the Flight Manual, and any FMS's, but while conforming to those, and proper training, flying a new certified type of the class you are qualified in, is not overwhelming. In some cases, regulators and insurers get carried away with obsessing about pilot training.
I do agree with what you are saying, and I was not suggesting that these were operational limits, but more, "group guidelines". I have come across a few groups who requested their aircraft be operated at specific power settings in the cruise (because they believe that this increased the longevity of their engine - which I was assured by a qualified mechanic was all rubbish...), this won't be in the POH / FMS. Or groups which had particularly peculiar checks (which turned out to be for good reason) which again were not in the POH.
I also agree with this quote:
The mindset to understand those systems, and use them properly is the same you will need to operated a prop or retractable gear properly, read and learn, then do.
But also understand that as a co-owner of a plane, I will want to know who is flying my shareoplane, and will want to know that they will take good care of it, and are more than capable of handling it. Irrespective of number of hours flown, or qualifications held!
I once flew with an ATPL holder, 10,000+hrs captain for a well known airline, I genuinely felt like he was trying to kill me during his first approach. So much so that I took over the controls and went around. After having talked about it whilst at a safe height, and explained why I deemed it an unsafe approach, he agreed it wasn't his finest approach... To his credit though his second approach was darn smooth!