Everyone has their own opinion on Wheel Spacers/Adaptors, I do not see a problem with them as a product as long as they are machined to a suitable tolerance, but unlike madluka I would not use spacers that were not hubcentric but that’s just my 5 peneth however the Loading on a Wheel bearing issue depends on what wheels you are fitting and their offset in my opinion if your 8 inch wide OE Wheels offset is for instance ET30 and you were to fit a different 8 inch wide wheel which has a offset of ET45 and you fitted a 15mm Hubcentric Wheel spacer the loading on the wheel bearing would be the exactly the same as you would be effectively be making your ET45 wheel into a ET30 wheel.
Even Vehicle manufacturers offer wheel options that end up giving a particular model a different overall Track measurement because of the different width and offset of the larger diameter Optional wheels you can upgrade to when you order your new car.