In my search for a fun and balanced army that can be played competitively I have come up with the following:

Warband 1: (255 points)
- Knight of Umbar on armored horse
- 12 Corsair Arbalesters

Warband 2: (168 points)
- Haradrim Taskmaster
- 6 Corsair Reavers
- 6 Corsairs with spears

Warband 3: (163 points)
- Corsair Captain with shield
- 6 Corsair Reavers
- 6 Corsairs with spears

Warband 4: (113 points)
- Corsair Bo'Sun with shield and spear
- 3 Corsair Reavers
- 5 Corsairs with spears
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Points: 699
Models: 47
Might: 8
Bow Limit: 12/43 = 27,9%

So, guys, what do you think? Looks like an army that could work to me

This looks like it might be a fun list to work with. I has some strengths in the form of reavers, crossbows and the Knight of Umbar but it has some major weakness that could be exploited too. Namely the low defense of your units all around. But the threat of taking extended fire from the crossbows will likely compel most lists to fight rather then shoot it out. I think that's a nice change for pace for the corsairs as opposed to just bringing the shadow lord. Your numbers are good but not really spam-ish. Maybe you could swap some models out here or there for some castellans of Umbar to get some more tanky troops but I think it will play well as it stands.

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

Thanks for the feedback. I'll have to make my mind up considering the castelans, while they would benefit the army in-game, the current list just completely has this 'raiding theme', something which units who are supposed to be the defenders of Umbar wouldn't really fit in to.

As for the low defense, I indeed added a full warband of arbalesters to win any shootouts. All bow units apart from other crossbowmen need 3 hits for every one hit I need against most archers (defense 3/4) so apart from maybe a Grey Company or Uruk force, I should probably be able to discourage any opponent to try and skirmish against me.

Seems like this army might indeed be good enough as it stands. That means at least one finished option for my choice of a new army .