For trade/maintenance workers, is union or non-union better?

I would prefer construction trade or maintenace workers answers but people with other careers/jobs are welcome as well here. I have never worked for a union company (I work as a maintenance electrician) but it seems the union companies near me offer better wages, benefits, vacation, etc than the non-union companies. It also seems the non-union companies I have worked at require you to be skilled/certified in many trades/skills like welding,HVAC, electrical, electronics, programming, mechanical (I could go on & on) but do not want to hire anyone to help you and pay very low wages. Union companies on the other hand seem to hire people for certain skills like electricians, mechanics, welders, plumbers, etc and they seem to get paid alot more. I do not know much about unions but what are the benefits of union compared to non-union (even in a construction trade). Why are there some workers that oppose unions? Fair days pay for a fair days work?