Each faction should have its own strengths and weaknesses, that make it better than others at certain strategies -hit and run, long range engagement, close quarters battle (rather than specefically good or bad at melee, shooting and movement if you get me). Creating the desired result is difficult. If a given faction is comprehensively unmaneovarable, then it limits the tactics and army selection for players using that faction, making games more predictable, and probably less enjoyable too. On the other hand its very boring if armies lack a unique flair and flavour. The idea of units from opposing factions having near identical stats, to me, would be totally offputting, and definitely the greater of the two evils
In summary it is good to give armies an "area of expertise" but not limit them to that. As you know, this is commonly achieved by giving their units good scores for certain stats or bad ones for others. However it's best to mix that approach with approaches like limiting access to certain units, e.g. Orcs are worse at shooting than humans, and so have a lower ranged skill. But you can give Orc players access to 0-2 units of "Ork sharpshooters" with high ranged skill. Unit synergy can also be a useful way of giving armies a forte, (though synergy should be subtle or you can end up with a snowballing effect that leads to serious balance problems).