I was a shop steward in the TGWU for 10 years and I have watched the erosion of workers rights over the last couple of decades.I wouldn't join a union anymore,most of the conveners and officials are self serving hypocrites these days and line their own pockets in preference to sticking their neck out.
Unions are people and they are only as good as its people (sometimes that can be just how good your rep or direct contact is).
They should, and largely do, give you bargaining power as a collective, which is invaluable.
It's a sorry state of affairs when any worker feels alienated from a union.