A quick snippet from the TUC research What do workers want? which has quite a lot of interesting stuff in it. Most interesting to a saddo like me are the findings on pensions on page 16 of the PDF. Union members are significantly more satisfied with their pension arrangements than non-union members. That suggests that unionised workforces have done a better job at hanging on to good pensions than those without representation.
Obviously this will be skewed by union membership in the public sector, but that still demonstrates that strong unions are likely to be able to retain good pensions. The private sector has been hit worst by the DB to DC shift, and that's where unions are weaker. And which was the first big UK company to shut its DB scheme to existing members? Non-union Rentokil.