First, I'm not going to get into the whole pro-union and anti-union debate. I'm just saying what goes on in my area.
Yes, union
sign shops do exist but are mainly in the northeast part of the USA afaik. Here in the Philadelphia area there are actually two
sign unions, the sheet metal workers (local 19) and the electrical workers (local 98). Phila has been a union town since colonial times BTW. Independence hall is very close to the old Carpenters hall in the Olde City section of Philly. Philadelphia
Sign Co. (actually in Palmyra NJ now) has about 300 employees and most are members of a union. No I don't work for them. NYC is another big union city. I can't speak for other areas.
While there is an "apprenticeship" program here (at least with the sheet metal workers) I can't really say that it preps you to do more than sheet metal work or install
signs. The first year of nighttime classes is almost exclusively devoted to welding. The union members I know are mainly in the installation end of things, though the fabrication guys at Phila
signs are union as well. If nothing else, they get good health care benefits. Plus the union will foot the bill for the crane operators license classes and a few similar things.
Oh, and the union dues (at least in Phila) are pretty reasonable at about $40/month with the sheet metal workers. Up in NYC, that's a completely different matter where the dues are more like $13,000 (yes thirteen thousand) a year. The employer pays that along with the pay rate in the $65/hr range. It costs a fortune to install
signs in a union situation in NYC but then it's a very expensive town to begin with.