The thing battle brothers lacks is transparency more than anything else. An even band of orcs will be a lot worse than an even band of bandits. At most the label is wrong, but the balance is fine.
1. Orcs are supposed to be physically stronger than humans. Orc young will hit harder than a normal battle brother, and an orc berserker can pretty easily kill all but the most prepared of your brothers. As a trade off, berserkers don’t wear armor, and are pretty easily to riddle with arrows before they close distance. Learn the strengths of your targets.
2. As for goblins, the solution has already been mentioned in this post. If you go after the little buggers at night time with one or two war dogs in tow, the fights can be ridiculously easy, especially with kite shields.
If anything battle brothers needs a manual filled with tips about enemies and strategies. For the most part, there is a strategy for dealing with most enemies in the game pretty easily if you know their strengths and weaknesses. The part that I find most brutal is the ambush system, where the game spawns an insane amount of enemy bands around your company to keep you from getting to an objective. Other than that, the game is pretty balanced. The superior physical nature of the orcs and the skirmish tactics of the goblins is what makes the factions distinct. Changing the composition too much would water down the experience and make certain encounters far too easy. Take some time to learn more about the game. I had the same complaints you did before I learned how to play.
As for the skeletons and the orc warriors, weapons have different stats that make them better or worse against certain enemy types. Skeletons don’t have vitals, and so arrows don’t do a whole lot of damage. Use smashing weapons. As for the orc warriors, use crossbows (which have a higher armor penetration stat) and weapons that crush armor. Most of your complaints come from a gap in knowledge. There’s a lot to learn in BB, and no way to learn it but trial and error.