There is no reason to get 'errors' if things are done correctly. I have used the transparency/UV/etch method for decades as an amateur. For single and double sided boards, results are as close to 'professional' as makes no difference, apart from not having plated holes.
Large scale PCB manufacturers use either screen printing or photographic methods essentially the same as hobbyist methods. 8000dpi plotters are readily available as are the transparent film rolls. Think around £6500 for that. Then, you'll need a good etching system. Hot spray etching is effective, around £1000 for a small unit. Of course, a PCB house will use bigger equipment than these for large scale duplication.
Add to that a big UV box for a few hundred quid and you can make single sided and double sided boards (without plated holes) very nicely. I can even manage simple 3-layer boards at home! With a lot of effort...
Want plated holes, or more layers? Send your design to a fab house, or invest a few tens of thousands in chemical processing, layer press, etc.
Design companies (professional) generally do simple development just the same as a good hobbyist might. For anything more, they will send the gerber files to a fab house.
If you want to have a look at some small-scale 'professional' level kit, take a look at Mega Electronics' site. I would love some of their kit, if I had the room and money.
https://www.megauk.com