I don't think any of the problems you state are legitimate.
The autoinjector he uses is a medically approved device used by many diabetics that can't bring themselves to casual stick a needle into their body and then inject the insulin.
1) Dosing is a big issue. Huge. Not just determining the correct dose but mechanically and reliably administering the correct dose. This is NOT a trivial concern. Both under and overdosing with epinephrine can be a very serious matter.
For dosing, it cannot administer more than was loaded into the syringe. Those with severe allergies can get approval and training from their physician to use a regular syringe and ampule to administer epinephrine to themselves (or a parent to a child). If dosing isn't a concern there, then it isn't a concern with preloading a syringe. Whether or not it injects the full amount is a factor of whether the device has been medically approved, and this autoinjector has been. If it is not killing patients by not injecting the full amount of insulin, then we shouldn't have to worry about its efficacy to deliver some other drug. So dosing is not a "big issue". As for the depth setting, that can simply be set ahead of time and glued in place.
2) Quality control in a device like this is essentially nonexistent. It might work but you can virtually guarantee that it won't always work. If it doesn't then that will very likely result in serious injury or possibly death. I work in a company that makes components for medical devices. The quality control standards are VERY stringent for very good reasons.
Quality control, as stated, isn't an issue, as the whole concept of using this device to operate a syringe that was loaded by the patient has been approved by the FDA and is in use.
3) Sourcing the medication. Sure you might be able to buy it but there are VERY good reasons why we have a controlled supply chain in the pharmaceutical industry. You are seriously rolling the dice if you buy outside the normal supply chain.
This is an alternative for people who can already legitimately get an EpiPen. If someone can get a prescription for an EpiPen then there are legitimate ways they can get prescriptions for ampules of epinephrine too. I already stated that physicians will write prescriptions for epinephrine for patients who will self-administer via a standard syringe.
4) The person who administers the injection is quite likely to not be the person who built the device. This raises a whole host of problems.
And? How is that any different than the person using an EpiPenis not the machine that made the device? You push the button and push it against the thigh, or vice versa. With an epipen you have to remove the safety from the back. Both require the same amount of steps and are obvious enough to use.
My concerns are more practical I do believe.
1) Sterility is a big concern. Any time you remove something from packaging it is no longer sterile. Medically that needle cannot be considered sterile. Further, I doubt the device he uses protects the needle from contamination in any way. It is designed to load and use immediately. Throwing the thing in a purse or backpack is a sure way to let dirt enter through the opening and get on the needle, etc.
2) Safety. The device appears easier to 'misfire" than an EpiPen, which has a cap that must be removed from the back of the device. With this, just through handling, I can see a person accidentally setting it off by pushing the button and bumping the bottom at the same time.
3) Size. This thing is bigger and bulkier than an EpiPenby quite a bit - across every axis it is larger.
4) Loss of medication. Prefilling a syringe and then storing it without an airtight cap could lead to a loss of medication. I suppose things like barometric changes in pressure could cause some medication to come out. I just don't know that surface tension is enough to keep all the liquid in the syringe indefinitely when the tip of the needle is open to the atmosphere, especially when it's being bounced around and vibrated, like riding in a car, etc.