In some of them the limit can be a bit too constricting, but if you don't have one then there's no real strategy to winning. All you need to do is spam units, whereas with a population limit you have to think about what type of army to build. Its actually one of the reasons why I lose interest quickly whenever I play C&C games nowadays.
In C&C it's managed by the resource income levels and counter-units.
A BH inf spam is powerful against tanks, but a single commando, several sniper teams or Hammerheads nearly negates it.
The problem with Halo wars for me is:
1. The base system. My opponent knows right off the bat where all my potential bases are.
2. The unit design. It's just completely out of whack. With Grizzlys beating banshees, Hawks beating grunt squads and ODST's beating wraiths, throws out the rock, paper, scissors style they were aiming for.
3. There's really only two effective strategys(3 for the covenant) 1. Rush, 2. Build large army of one unit and just attack or 3. Build two scarabs.
It's always the same for the UNSC: Grizzlys, Hawks, ODST's or the Covenant: Jackals, Hunters, Vampires or 6 scarabs.