We are a very small two man operation that runs a 6 color manual press. We do on average around 1000+ pcs. per month. We also sell signs, embroidery, printed materials, and promotionals so screen printed garments is only a part of our business. I would say the average job for us is about 50-100 pcs. When we score larger jobs 500 pcs. and over, that are better suited for an automatic printer, we always sub out. It's just easier and we need our time to keep up with other things. However, our local contract printer is only 15 minutes away, does great work, and allows us amazing profit margins, so for us it's a no brainer. I suppose it's not for everyone, but if I were you, knowing what I do know about screen printing, albeit my knowledge is very limited as I'm still very new to this business, I would sub it out if I could just for the stress relief alone.
Screen printing has a huge learning curve. Anybody can pull off fairly decent spot color prints with a little practice, but perfecting 4 color and simulated process prints requires years of knowledge and practice. I think people are just warning you to keep you from jumping in over your head. As Lon Winters once told me, the average screen printing job has countless variables. (Many, but not all of which, were previously mentioned in this thread.) If your job has, say...thirty variables and you fudge up one, you might still get a fairly decent print. But if 3 of those 30 variables are off, your job is seriously flawed...in fact, screwed. Just food for thought.