Thoughts?

Knight of Umbar (armoured horse)
6 Castellans of Umbar
6 Haradrim Warriors (spear)

Corsair Captain
6 Corsair Reavers
6 Corsairs of Umbar (spear)

Haradrim King (horse, bow, war spear)
6 Watchers of Karna (bow)
6 Warriors of Karna (bow and spear)

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Watchers and reavers are totally broken so your list seems pretty good !

I like the list a lot, it's got a nice mix of troops that can all deal with different situations. You've got plenty F5 which will be a pain for Elves to deal with and multiple attacks models who'll have a field day against D5 or less and give you a better shot against higher D models purely due to volume of attacks. The KoU is a good choice although I do prefer him on fellbeast to make real use of his combat abilities since his magic isn't as strong as the other wraiths.

The great thing about Harad and Umbar is that you've got lots of options. I find them to be the most versatile army in the game. Below is a similar list in size but with different elite troops, this one will struggle more against Elves due to lack of fight 5 but it still has a F4 line with re-rolling 1's and 2's or plus one to wound on most of your troops (The front line warriors can feint if backed up by F4 serpent guard who get re-roll's anyway due to poison). It also takes advantage of the 50% bow limit that you have access to without the Corsairs and a Fellbeast to provide Monstrous Attacks (can use KoU on it if you prefer). I wouldn't necessarily say it's a better list but if you've not bought anything for the army yet it (along with your original list) shows the variety of troops that you'll have at your disposal and different ways you can take a Harad and Umbar list.

Warband 1
The Betrayer with Fell Beast;
6 Haradrim Warrior;
6 Serpent Guard

Warband 2
Haradrim Chieftain with Horse; Bow; War spear;
6 Warrior of Kârna with Bow; Spear;
6 Abrakhân Merchant Guard

Warband 3
Haradrim Chieftain with Bow; Spear;
6 Warrior of Kârna with Bow; Spear;
6 Watcher of Kârna with Bow;