The more I play BTS and the more I read these boards, the more I realize how many angles there are. To me, the musketer does seem rather weak, but a +25% to vs. melee seems a bit too much. Musket men have a short shelf life for me, I make it a point get rifling shortly afterwards as I consider rifling to be one of the top 5 milestones (railroad and three others I haven't decided on making it to the list.)
So before rifleman I usually have stacks of pikeman, knights or preferably crusaiers, trebuchets, and crossbowman I'm not so much into macemen like many people on the forum.
BUT consider this. If your greatest unit is knight and you discover gunpowder, having squads of knights/musketeers could be great for cleaning up pesky pillagers in your territory (or perhaps being the pesky pillagers.) The pikeman will lose to the musketeers and everything else will lose to the knight. Not enough of an edge to likely be a good invasion force, but with enough trebuchets you should be good to go. At any rate mobility can be huge, especially when infrastructure is not completed or pillaged or if an invader has you outmatched number wise for the time being.
The same thing applies to the impi and horse archers. Axemen shouldn't be a problem, especially if you have some chariots left over, and you can go toe to toe with any swordsman, plus you'd have a huge bonus against catapults.
.There's always an angle, but obviously some UU and Ubs are better than others.
As for realism? I'm not quite sure, I don't know much about battles in the musket era, but armies weren't completely composed of musketmen, and it wasn't just because of a lack of resources. Many a samurai and archers killed many a musketmen through out the ages.