I’m not the foremost expert on this but I think the main reason is the absorption, and that over time your body will absorb less of it. My understanding is that gel is better for libido, and injections for everything else.
On the bright side for you, the allergic reaction tends to be to the glue that holds the patches on, not the T gel itself. My own worries about the gel were all about contact with others and the pain of trying to keep it off clothes, etc.
To be clear, I was terrified of injections, even to the point (when I had to go on them) of trying to convince my doctor to let me do subcutaneous - he didn’t want me to and I didn’t. The idea of a “deep muscle injection” just made me feel awful inside and I didn’t want any part of it.
Once I got past the roadblocks, it’s pretty much like doing anything else. I’m paranoid about cleanliness and infection so I work in the same area every week and I use a brand new laundered towel to lay everything out on. And I alcohol the area as well as my fingers before I work with everything. I’ve done so many now (two years worth), it no more interferes with my life than brushing my teeth. But again, it’s the benefits I have felt from “optimizing” which outweigh anything else. I’m talking a total life transformation. If you get that from gel, go for it, but I guess my point is please don’t sweat this idea of “injections the rest of your life”.
We all went through that thought process at one point, and for the people who have success with the injections, it ends up being a fairly insignificant concern to the point of irrelevance.