Most of the medium ones aren't even skirmishers. They're regular cavalry with javelins. Again, the proper javelineers - like Hippakontistai - have 5 or 6 javelins. The problem here is not the units, it's the strange expectation people seem to have that medium units which aren't dedicated skirmishers aren't able to perform just as well as the dedicated units.
We've already given them an almost unrealistically-long range. The animation is as "efficient" as it can be without becoming unrealistic.
No infantry unit should have very_hardy, it's effectively "never tires". Which removes that element of balance altogether. Foot skirmishers (again, real skirmishers, not javelin-armed line infantry) are already faster than line infantry and have better stamina and lower stat_heat as well.