As always, mercenaries are still quite situational due to the randomness. Every time you spend 200w on the tavern/monastery, it’s like a gamble. Imagine seeing ninja & arsonist & zouave (not useful), or gatling camel & mameluke & lil’s bombard (all age 4).
Another problem that comes specifically with DE is that many new mercenaries are added, and some have overlapping roles (fusilier and askari, mameluke and sennar horseman, black rider and zenata and kanuri, etc.) and there are now enough age 4 mercs to make your all three slots age 4 (which essentially eliminates the strategy).
The resource trickle is probably one good change that encourages people to at least build this building, but mercs themselves are still not used if the ideal ones do not pop up. And randomness is rarely viable in any serious gameplay.