Numenor are a middle of the park army. You should think of them along the lines of a Minas Tirith or Moria Goblin army.
Their main drawbacks are their limited choice of troops as you have mentioned. They are capable of winning games though, and you can get enough variety there to change the play style a little.
They are definitely capable of winning and I liked the models so I got 12+ each of bows and spears. I played against an Uruk army with crossbows and beat it, not against the strongest player, but thats fine as Isengard is known to be very competitive and quite forgiving.
My list is Isildur, 2 Captains, 12 Sword + Shields. 12 Spear + Shield, 12 Bows for 500 pts. You can take out troops here and there to add horses and shields which doesn't sound like much, but it does change things a bit and makes those heroes really important. I love Numenor, so I enjoy playing it, but it's up to you. I'd suggest getting them because they are good sculpts
The strengths are that they are cheap, yet have higher FV than most opponents and as you can normally outnumber people, you should be ok if the dice are average. A banner can be useful, but playtest to see how you go. If Numenor had the cavalry option, bodyguard and an option to upgrade to S4, D6 or better bows, they would be nasty!