My experience over the years with fleets, says yes, GM's are not that well built. They try to engineer a dollar out of every part, and make vehicles with the same defects year after year. They also have excessive torque management and outdated powertrain technology that they are reluctant (and usually last) to abandon. Its a company with decent engineers, who are betrayed by bean counters and myopic leaders.
Ford used to produce a lot of garbage, for a long time, they are now kicking GM's ass in powertrain technology. GM's can get decent mileage, but crappy throttle response brought on by excessive torque management spoils the driving experience, for those of us who value the nuances of a well engineered vehicle. GM is the only brand I will not buy or recommend. Toyota is in a funk now as well, with outdated (10 year old in some cases) powertrains that do no compete in efficiency.
If not for all the repeated garbage I see fleet experiences with GM's that I do not see in other brands, I may not have as negative an opinion of them. For GM, good enough to get it out the door in may cases is, well, good enough.