I find it weird as well - uruk troop tress are capped at low levels like orcs, i.e. five of them are less useful than a top tier elf, Gondorian, or Dunedain, but they don't benefit from the orc party size bonus. To be honest, the only Uruks I use are the Isengard trackers against Rohan, and that only if I am playing orc/uruk exclusively. Otherwise, I use a mix of Harad and Umbar archers.

As for why your Uruk recruitment is dwindling, I may have an idea. If you are recruiting only uruks, and leaving the orcs as available recruits in the tower, random recruits are added, but only up to a point. If you keep leaving orcs in there, eventually all slots will be taken by them, and you will maybe get some uruks only when the slots are increased.

So do what I do - hire everyone, and then either stash them in your reinforcements, if you are at home, or put them back in garrison. As money becomes pretty cheap if you know what you are doing, there is very little cost to it. And if you get the "Barbarian champion" and "Pit master" traits, I think that there's no cost to transferring recruits to the garrison, i.e. you get back all your expenses.

It really sucks , we should have a possibility to create a horde of uruk hai , to hire more of them - uruk hai worms die in 1 -2 hits and are rare....I always wanted to have my own uruk hai band of 100 men but its impossible...I knew the answer on 2nd question already , mod authors should really take a look . Its not fun when your uruk hais die and you cant reiforce the losses and you are forced to take isengard orcs..and why uruk hai and uruks from mordor dont give us the "horde" bonus to have +party size I have no idea..its a shame

Oh, you can still create a horde of Uruks, you just need to get really high ranks. Much harder AND RISKIER to achieve with the Dark side at 149% difficulty, but definitely possible. Varyag heavy skirmishers are not Noldor mounted archers, but with a combat character who has the Berserker trait, you can still pull off 400 rank points from some battles against the Beorings. Too bad that you can charge them to Moria.

I got highest rank in isengard as uruk hai , and still I struggle to gather some amount of uruk hai

Ranks in other factions help. My elf character had nearly 200 party size by the end, without ever investing a single point in Charisma or Leadership.