A lot more horde players use war mode, simply because their faction is so dominant in population. The 10% bonus is worth it for them, as it essentially makes no difference to them in terms of gameplay. I personally play Alliance and war mode was ok for the first 2 weeks of BFA launch. It was reasonably balanced in terms of population and you could get your quests done in a decent amount of time.
However, shortly after this period the horde population started skyrocketing, and warmode just became a huge time sink. If I wanted to complete a champions of azeroth world quest, there would be 10 horde for every 1 alliance player, at a minimum. So you had 2 options, 1) slowly chip away and get a percent here or there while you die repeatedly or 2) try to join a group to complete the quest. Even joining a full 5 man alliance group though often resulted in dying repeatedly as alliance were still heavily outnumbered. It just turned doing a 3 minute world quest into a 20 minute chore. So the bonus was no longer worthwhile.
Warmode is also hilarious in terms of summoning for a dungeon. If you are an alliance player, try going to the Tol Dogor island with warmode on. For some reason, there are just 50 horde players outside that dungeon 24/7. You can barely enter the dungeon without dying, let alone summon anyone. So warmode just becomes tedious.
Honestly, the intention behind warmode was decent but I don't really know what they expected. The result was easily predictable. They would have to balance the faction populations if they want warmode to be a functional experience. And at this point the factions have been imbalanced for so long that I just don't think it's possible to fix it.
