The problem is that OEMs/ODMs don't take you seriously unless you are building in large scale. So for small scale, look at perhaps thousand per device -- so you can get access to the production line at an ODM that does small volumes. But forget about leverage with suppliers.
And the investment in certification and related paperwork for N number of countries... just does not make any sense.
If your battery is special, you may not even be able to ship it. Batteries pack a ton of energy in a small package (some say as much as a grenade) and can be unstable. So there are restrictions on airfreight. Ship by boat and wait 3+ months.
Some countries will not let you import devices without chargers; and are really anal about charger certification.
There's a zillion details like these. Each of them can be overcome... each of them at some cost in manpower, calendar time and cash. But often you suffer a thousand very expensive cuts _even when you have folks with a ton of hw experience involved_.