Having worked at museums with unions and without I
can only say that both have their up and down
sides.  The union insured that staff would want to
stay employed longer and contributed to the
feeling that we were a family.  It also made it a
lot easier to work for low wages because of the
benefits such as health insurance, overtime, and
sick pay guarantees. On the other hand, it also
insured that unmotivated, uncaring, poorly
trained, and just downright lazy people were able
to have a job for life.  This may be alright for a
department store, but when everyone is expected to
pitch in during an emergency, it didn't work.
Cries of "it's not in my job description" were
commonplace.
For the most part, I discovered that the people
most covered by unions in museums were the guards,
cleaners, and secretaries, not conservators,
curators, educators.
J chin