Whistleblowers, company records, and reprisals against previously valued employees.
In the good old days management was quite open about their opposition. They'd even hire thugs, private detective agencies or off-duty police to beat up unionists, or have the governor call out the national guard. There used to be riots and shoot outs.
Nowadays such open antagonism is politically incorrect, but the calculus is the same: reduced expenses and obligations = increased profits.
No more riots, but union membership is down, and those that exist are effectively subverted by right-to-work legislation. Worker power and security is undermined by more subtle means: "independent contractor" status, part-time hours just below the benefits level, pension replacement wit 401-K programs, and the accumulation of innumerable little policy changes and regulatory tweaks -- unnoticed by the general public.
If you don't remember what you had you're not likely to notice our descent into serfdom.