Breast implants incision sites, how difficult is it to insert gummy bear implants via the armpits?

Gummy Bear, or form stable implants, are much more difficult to place from the armpit because they do not give as much as the softer implants. It can be done but you will have a much larger incision, usually. I would recommend either the infra-mammary crease or infra-areolar incision sites normally for form stable implants. Talk to your surgeon in detail and make sure you are happy with the answers you get. I know you are afraid of a mark on the breast but if done correctly and using fine technique that can be minimized.

Many patients will struggle with the choice of access incision. While there are many options, it is important to understand the limitations of each, especially as it relates to form stable devices.The choice of incision in the setting of breast augmentation is one which is made jointly by both the patient and the surgeon. Surgeons typically have their preference as do patients.
One thing to consider is the implant type and size as it relates to incision choice. Silicone implants typically need larger incisions than saline implants (due to the fact that saline implants are placed first then filled). The difference is subtle (on the order of  a centimeter) but is worth considering. The need for a larger incision is even more crucial when considering form stable shaped devices as they require a still larger incision. The manufacturers recommendation is for a generous IMF incision (although many surgeons have moved beyond this).
Additionally, there is data which indicates that there is a slightly elevated risk of capsular contracture. This is thought to be a consequence of bacterial contamination which is attributed to traversing the glandular tissue on the periareolar approach (nipple is connected to the outside world and the ducts which are then linked to the glandular tissue).With regards to your specific question, there is always a risk of keloid scarring anytime an incision is made. Fortunately, there are options for managing them including silicone sheeting, steroid and 5-Fu injections.
That being said, both approaches are safely employed by many surgeons. As always, discuss your concerns with a board certified plastic surgeon (ABPS).