I think it's hard to be a job shop and make your own products as well. I know we couldn't do it, so we don't ever job shop. Here's why (small company edition, the rules change when you are big):
Making your own products, you need deep knowledge of a limited breadth. You need specialized tooling and specialized processes. If your product was such that you could have it made anywhere by any shop, it's a non-starter anyway, because there will be cheap knockoffs immediately.
As an example, a product I will be releasing shortly has a plasma electropolished titanium assembly. Plasma electropolishing is pretty specialty, titanium electropolishing is pretty specialty, and there are some unique characteristics to the part that make it tough. Any job shop is gonna look at that part and go "yeah, no thanks" especially at the cost per unit I need. We had to build an entire system around that specific part.
A product shop can do that, because at a product shop, you tailor your designs to your ability to produce, and you tailor your ability to produce to your designs.
A job shop has the opposite needs. Limited depth (you can always no-quote something. you can't no-quote your own parts) but they need to be able to handle a much wider variety of parts and do it cost effectively. A job shop can't spend three weeks tweaking a part (they may never see again) in. They have to get it right the first time. They need to have all the tools, not just some of the tools. They can't have major downtime like a product shop can. They can't say "this 304 stainless batch is crap, I'll just make it out of 303."
But a combined shop has the worst of both worlds IMHO. Either they're designing products that can be run with commodity capacity, and they'll get knocked off as soon as they launch, or they have a bunch of specialty capacity sitting idle while they run a batch of 10,000 spacers.
I'm sure there are counter-examples, but that's why job shoppers feel like you can't make money on products, and product people feel like you can't make money job shopping. They look similar on the outside, but there isn't as much overlap as it seems.