I've finished Jabba's Realm using Nemeses, and I found it to be pretty powerful. I think there are a few things about some of the cards that aren't immediately obvious. Firstly, one of the powerful things about the class is that it gives you free threat. Thus, one of the goals is to make the best use of this threat. As Serox points out, that means you want the best health to threat ratio you can get. But, there's a second not so obvious factor. A lot of missions allow for the Imperial to get a victory, often by interacting with things. I won't go into spoilers here, but what I will say is that I picked Greedo and the Grand Inquisitor, and in at least three missions I won because of their movement and survivability. Greedo is weak later, but in earlier missions he's an terror especially against melee parties, and when movement is a factor. Basically, having the free threat allows you to take more actions per turn, especially if you hold off a little and bring them in a bit later when you've got enough threat to bring the villains with a few buddies too.
Secondly, some specific points about some cards:
Punishing Force - This card is amazing, and I think even in a villain buffing build it should be included if possible. At least in Jabba's Realm I found I had enough XP for this, I'm on the Leader, and a 4xp card. When rolling two dice, you are usually using two or three figures. I found I almost always used it in these situations. For instance, it allows you to take shots that are riskier in terms of accuracy, or to reroll your dice in response to the defence. I found it was averaging an extra 3-4 damage in a round, or applying status effects that otherwise would have missed.
I'm on the Leader - You can almost always get this to work. When you are focusing someone down, they often get wounded in enough time for you to use this on the next person you are focusing. Often, even when you are focusing, you can't attack that person with everyone, so you just target someone you can use the +2 damage on. In my campaign, it often ended up forcing characters I wasn't focusing down to act sub-optimally, because they had to think about exposing themselves to damage. All in all, I'd say it's actually pretty easy to use, and I ended up using it pretty much every round.
Prepare the Ambush - The skip a turn ability is occasionally useful, but the +2 movement points is amazing. I think what I didn't appreciate when I looked at this card is that often when you get troops spawning, at the start or through deployment, the figures are either given to you in sub-optimal places, or you have to put them in sub-optimal places, OR the rebels are focused on the goal and so are running through the map quickly and leaving some figures behind. +2 movement to your first group usually allowed me to attack with figures that otherwise wouldn't have got an attack at all. Jabba's Realm especially give you Gamorreans a lot, and Prepare the Ambush meant that I could always attack with them. It's also great against heroes that can stun - you go from having no move, maybe an attack if you can get into position, to having a 2 movement point move and probably an attack. I was against Murne, so this happened a LOT. Some missions this can be really useful in too, one side mission The Grand Inquisitor was able to double move for 12, which in that mission resulted in an Imperial win.
Ringleader - I found this good in the mid game, not so great in the end game where you have to be a little more careful. But, I activated it enough that I think it was worth it. It's great on maps where you get an optional deployment at the start and can have your villain out straight away.
Both 4xp abilities - Both kinda situational - Indomitable is good at helping your villains to absorb more activations than they should, and useful on maps where they have to kill villains. Devastating Legions sounds good, but in practice it's quite hard to get the condition to happen often enough. Especially if you have ringleader, your villains often survive by getting into annoying places where they are hard to attack, so they are often not close enough for it to work. I went with Indomitable, but I'm not convinced it made a huge difference. The sub-4xp abilities in Nemeses seem more impactful.
Some thoughts on build:
It's a little hard to judge this totally, because Greedo helped me win early missions, which resulted in me having 14xp by the last mission. But I think something like this is good:
I'm on the Leader -> Punishing Forces (get punishing forces first if you end up getting 3xp rather than 2) -> Prepare the Ambush -> Ringleader -> Indomitable -> Whatever else you can afford, I'd suggest Leave Them To Me, but I'd use it to cheat out a free cheap Villain rather than your "main" villain. Preferably when you have an influx of other troops too.
Looking at the two builds you suggested Serox, I'd stay well away from the pure buffing route. The real power in Nemeses is in I'm on the Leader and Punishing Forces, Leave Them To Me is very difficult to use well. Your troop buffing build looks better, but I really would get Prepare the Ambush in there. I would say that most rounds it enabled attacks I wouldn't have otherwise got, so you could think of it being worth 3-4 damage a round, but that's considering that I had Murne stunning things. Probably more realistically 2 damage a round - still, that's as good as I'm on the Leader!
Hope that helps!