Training members for an unskilled job doesn't really improve the value delivered by that worker. You can see why employers would not see that as a benefit of a union. Skilled jobs like the trades are already differentiated based on skill and reputation through the contract bidding process.

Is that relevant in our race-to-the-bottom world where every last corner is cut? I am strongly pro-union, but I don’t actually see corps seeing your point as a positive.

International unions would prevent that. Can't cut corners if everyone you hire is united to ensure everyone is treated equally by the company.