I can't speak much to manufacturing domestically (Canada/US), but I have a lot of experience dealing with manufacturing overseas (China, Vietnam, Indonesia)
I did my first product 4 years ago, and it was an absolute nightmare.
Error No. 1 I found my manufacturer on Alibaba.
They spoke english and had a very "yes, of course we can" attitude to every request I had. There was nothing I could ask for that they would say was not possible. The communication was pretty responsive initially, but once they had my deposit, it was damn near impossible getting responses in a timely manner. I eventually learned that 95% of the "Factories" you are dealing with on Alibaba are just english speaking agents working for many different factories. They have no clue what they are talking about in regards to manufacturing, and they will promise the world in exchange for a deposit.
Error No. 2 Unrealistic tolerances in China are rarely discovered until it is too late.
When one of these Alibaba agents yes "yes", it means it MIGHT be theoretically possible. Your tolerance requests might be possible, but increase production cost by 30%. Your tolerance requests might be accepted and then just ignored.
Error number 3. Late stage QC
Quality control needs to be done from the very beginning. I would have saved so much time and money if I was working with someone that knew the ins and out of Chinese manufacturing from the start. My initial prototyping phase was almost useless because it was near impossible to manufacture from the start. If I knew what the factories limitations were from the beginning, I would have avoided a lot of headache.
Error no 4. IP protection.
Manufacturing everything at the same factory makes it way to easy to steal your IP. No single factory should have access to all of the information, regardless of how good of a relationship you think you have. The most effective IP protection is to manufacture individual parts at multiple factories, and have them assembled at another factory. This increases the quality (more specialised), lowers cost, and protects your IP. This is how all of the tech giants like Apple and Microsoft do it too!
My first product was a total failure. My second product was a near failure. 3rd and 4th got better. Then I started helping some other friends and family tackle their own products.
Fast forward a few years, and I'm working with a Canadian run manufacturing company over in China with 400+ products under our belt. Learning the language, culture, and manufacturing process in China has taken us years, and is the only reason we have been able to pull it off.
Its still not a piece of cake dealing with the factories over here, but we have built solid relationship with our primary 30-50 factories that we use for dozens of products every year.
Feel free to bounce any overseas manufacturing questions off me!