All I can say is don't put very much stock in those "user ratings". Typically only those either very happy or very angry with their purchase will write one. Also those who fancy themselves writers and just like to write those things.
TOO many times you do see things like a very poor rating over something trivial like your "no spare battery" example. Or where the buyer simply did not read well and got what he was supposed to but not what he was expecting.

Regarding a "Leading consumer magazine", look to see who the publisher is. Chances are it is owned by the same folks that own the big store. It is a classic marketing technique to publish your own "independent" report under the guise of being "independent".
If it's not Consumer Reports I would not trust it.

About gensets, DO NOT let the mega-store install it. Quite typically their sub-contractors are let's just say, sub-par.

Call around to places that sell them and see if they can recommend an installer or possibly a package deal like the big stores.
It will generally be cheaper to go with the big store but IMO it is NOT worth the little cost savings.

I do know for fact that Generac's after the sale customer service is terrible.

I would much rather have a Kohler than the Generac, "consumer ratings" notwithstanding. While the quality of Generac units has improved some, they are still often called "Gener-junk" by generator service techs. When you have to fix the set, (and you will, regardless of brand) Generac parts are often so expensive that it makes more sense to buy another unit.

Keep in mind the sets you are looking at are designed for "optional standby"/non-critical use, regardless of advertising hype. You are not buying the same quality as that of the unit that sits behind your local 911 center. If an outage is inconvenient, a residential grade set is OK, if it could be life threatening, all of the "big three" have commercial grade 15kw sets available, at perhaps twice the cost of the units you are considering.

Having said that, my order of preference is Onan, Kohler and "everything else". The Kohler and Onan in that price/size class are about the same quality, I wouldn't hesitate to use either brand for optional standby.

I will input that I have the Generac 15kw installed about 2 years ago. Installed it myself after ordering from Amazon of all places. Generac sent out a certified installer to inspect the install and do the startup run to meet warranty requirements. Cost me about $150 for the service call. The tech was friendly and very informative. I have not had to have any other service since then. The unit functions great and runs its self test every Saturday. It is due for its 2nd oil change in a few weeks. I bought a 6pac of filters from Generac when the unit was new but Home Depot sells them now for cheaper than I paid.

The oil changes are very easy as the design was well thought out. I do need to touch up the paint at some corners where the weed wacker has hit it and therefor some rust spots.

For the price, I am very pleased with the unit.