Correlation != causation. There are a ton of differences between the US car industry and those in other countries, unionization is just one factor.
As a counter anecdote I’d point to Boeing’s non-union facilities, which have produced notably less reliable airplanes than their union locations ever did.
Boeing actually offers a fascinating direct comparison. 787 Max has historically been assembled in two locations: unionized Renton, WA and non-union Charleston, SC. The Charleston planes were notorious for needing rework at Renton before they were airworthy.
But the conclusion is muddied by the fact that Boeing has been making planes in the Seattle area for a century, so the talent pool is larger and more qualified than those they could find in or persuade to move to Charleston. Also, the whole Charleston move was one of many drastic cost-cutting efforts, including the spinoff of Spirit Aerospace that ultimately led to the door blowout on the Alaska Air MAX 9.
As a counter anecdote I’d point to Boeing’s non-union facilities, which have produced notably less reliable airplanes than their union locations ever did.