I used an external frame pack for over 40 years and it worked well. I still use it if I have to carry a lot of weight (for example if I have climbing gear). I used it in all conditions, including off-trail and over some technical terrain. It is a matter of loading it correctly, having it fit perfectly, and practice. When scrambling, put the weight mid-back, centered and close to your back. Use a sternum strap. The hip belt design has a lot to do with the stability of the pack. You may have to try several different designs before you find one that works. Any pack, internal or external frame, that is heavy will pull me off center when climbing, so I aways carry a line to haul the pack if needed. The advantage of an external frame pack is that it carries heavy loads more comfortably. If you use an extension bar up top, it places the load directly over your center of gravity.

I have pared down my external frame pack to just under 4 pounds, including extension bar. I use an internal frame pack when I can keep the weight under 35 pounds.

So there is nothing "wrong" with an external frame pack but it does take more practice to learn to use it properly. In fact, I did three 2-3 week winter mountaineering trips on skiis with an external frame pack loaded to 60-70 pounds (when I was much younger!). If however you can keep your weight low, internal frame packs offer a lighter option and are a bit more friendly to beginners.