For me:
The usual cuts, bruises, lacerations, burns, soft tissue injuries. Always have a proper cuts kit.
The worst field treatable thing I have had was actually a bit of frost-bite in the arctic. On my face. That took lots and lots of care and attention for days not to let it get worse.
The worst at all was multiple injuries including dislocation - from a climbing accident. Stabilise and transport. That was IT.
What I have witnessed and been able to resolve with Field Medicine (either resolve in the Field or stabilise for transport):
Resolve:
Heatstroke
Hypothermia - stage 2
Saw cut that needed Kotex - butterfly strips - glue
Jungle rot
Salt sores
Frostbite
Severe allergic reaction
Amoebic dysentry - resolved enough to be able to walk out - HOSPITAL right away after that. Long term nastiness.
Stabilise & transport:
Compound Tib/Fib fracture
Spleen & other internal organ bruising after a fall
Concussions
Field Medicine really is actually quite limited:
If you can deal with it in the field it was probably relatively trivial.
Cuts etc OR things you need to deal with NOW so that they don't get beyond trivial.
(Infected wounds, frostbite, etc.)
Anything major and you are doing Life Support and waiting...
So when you think of that.... WHICH of the things that are in your FAK will actually help?
I have:
Cut/Burn gear
Mild, Moderate and Serious painkiller gear - this is very important. Not just for comfort, but for functioning at all
Serious anti-allergy gear
Asthma inhaler (not for me but always good to have)
Anti-diarrhea - diarrhea is dehydration. And dehydration is death.
Travel sickness tabs
Skin lube
Blister gear
Duct tape
Piece of tubing... Many uses from tourniquet to emergency hydration
Eye drops!!!
If in more remote areas then add:
Anti-biotics
Emergency Toothcare
That is it. If you can't handle it with this level of gear, you are unlikely to handle it in the field anyway....