Have you guys had to outsource any work and found it difficult to control?
or even had the other company trying to muscle in on the client after the job?
Jef Lippiatt , Owner at Startup Chucktown
I have seen this in some of my previous work experiences, however, I think it vastly depends on how well you vet or verify the people you outsource work. I believe that you you conduct your due diligence you're more likely to find a good long term partner. The next part is working on that professional relationship in an ongoing manner. Constantly working on the relationship helps surface areas of frustration before they lead to problems (like them trying to steal your clients).
Speaking specifically about one of my ventures, children's books. I can say I did my due diligence in getting a solid illustrator. I've worked with him on a platform for about a year. Then we took it off that platform so he could give me better pricing and make a bit more money himself. We've been working together for almost 2 years now and it is a great relationship.