I know that Adunabar can get by with its Dunedanic roster, but I find the temptation of all the neat Cultic units to be too strong. When I play Adun., I go for Olog-Hai and Uruks as soon as I can, and Wargs too ... eventually.
Of course, it's a good idea to keep training Mannish units as well, because any Orkish units below Champions are easily beatable by most other factions. One exception here is Rhun, who you're bound to come up against sooner rather than later: their infantry units are mostly weak, so it's actually pretty cost-effective to send some low-ish tier Orcs into the East - supported by some of your basic Mannish units, like militia, swords, bows, and cav. Keep your Longbowmen and Royal Spearmen in the West, where you'll need them against the RK. Of course, if you have a few Mannish elites to spare, they can do a great deal to help defend against Rhun.
Even if you plan to go all-Orkish, though, you'll still need some support units - especially the Royal Longbowmen, and cav (eventually you'll get nice, scary Cultic cav which will be a nice upgrade). Swords of the Shadow are good infantry of the line, but I never train enough of them to make up the majority of my armies - usually I have a couple units flanking a line of Uruks, either Uruk Swords or Uruk Spears. IIRC both Uruk units are fast and/or have good stamina, making them surprisingly mobile additions. The big advantage with these Uruks is their unit size and decent defense, which means they can do surprisingly well in an "anvil" role, while you circle around to warcry & charge with Champions or cav.
Down the road, Adun. gets some nice niche units - Uruk Berserkers and Halberds. Berserkers are what you'd expect - some people don't have much use for 'em, but I like to have a couple units for the fun of it, and to cause chaos in low-tier enemy lines. Halberds are great IMO, but they have to be used in sufficient numbers and in the right situation. I have had great results using them to bar river crossings in the northern Anduin against Rohan. Just let the cav come across, and chop 'em down with your Halberds (after they've been shot up with your Longbows, of course). I haven't used them against infantry-heavy armies, but I imagine the results would be poorer, which would make sense.
The "beasts" are a lot of fun too. Olog-Hai are the signature unit, and great against RK, which can usually field stronger infantry armies than you. Send in the trolls, let them smash a bit, and then run them around through the enemy ranks before they get surrounded and cut down. In a battle line, you want them on the flanks so it's easier for them to cause chaos without trampling through your own guys. Remember that they need support; a great tactic is to follow up with a charge of Shadowriders (or other swift units) immediately after the trolls have sent the enemy flying. Meanwhile, move the rest of your army to engage, and with a little luck you should have a rout.
Wargs are a very late unit, but good against the factions you'll be fighting by then - probably Rohan and Harad, both of which field cav regularly. They also add a measure of mobility to your (somewhat) footbound forces.