Just a few remarks for a new player :
1) heavy fliers (things like dragons, or hordes of flying large infantry or cavalry like the Basilean Elohims or the elven Drakon riders) can be very powerful, but they are also very expensive in points, and while there is multiple tactics to counter fliers, it's usually easier to learn to use them than to learn to counter them.
In other words, I usually adivise beginners to try their first games without such heavy fliers and only add them once both players have masterd the base game without them.
That's not to say that they are unbalanced, it's mainly a learning curve problem, and it can result in boring games where the whole game depend on the big flier being correctly used or not.
Light fliers like gargoyles are however nice to learn the flying rule without them dminating your first games
2) Note that even when some builds are more popular than other, most armies can be played in very different ways and still be perfectly viable even at tournaments.
For example with undead you can both make horde armies with lots of cheap zombies and skeletons and cheap to average offensive units like zombie trolls, or you can go full elite with only revenant, vampires, werewolves and wraith, or you can do a mix of both.
3) Even with the previous point, it is however true that some armies do have some elements that can make them feel different from others
Among the more extreme are :
They have almost nothing in ranged attacks, and their fast options while decent are not as varied or as fast as some others armies.
You can still do an orc army with a decent number of fast units (fore riders, mounted heroes, heroes on slasher, especially the flying one), but gameplay wise orcs tend to be a very "in your face" army with a very direct style of play. They can be powerful, but some players might find them slighty boring *compared* to most other armies.
I find that they play somewhat like the Warriors of Charos in Warhammer 8th edition
- Undeads (both the core Undead list, and Empire of Dust in the Uncharted Empires supplement)
Those armies both have a lots of shambling units unable to "Move at the Double", but with good access to the Surge spell.
For beginners, surge might seems like wasting your precious spells to partially negate your speed problem, but in fact Surge is among the most powerful spells in the game if you know how to use it.
for some example of Surge tactics
This will also be useful for other armies that have access to Suge and their own shambling units (mainly elementals), but where those other armies can chose to use Surge or not, with undead armies you are almost sure that you will want it due to the almost universal presence of Shambling units.
4) While the meta encourage fast units, in KoW
the simple number of units is important too, so even a fast army might want some cheap options to get a decent number of units drops (if you don't you might find yourself outdeployed, easily flanked, or lacking in units able to occupy objectives).
This plus the new unit value system for controlling objectives means that even in this meta infantry or other cheap and weak units have their uses in the game, if only to hold objectives without wasting too many points, as chaff to sacrifice in order to delay a stronger unit and many similar tactics.
A classic beginner error is to only take units that look powerful on paper by themselves without thinking of your whole army.
Even if for example you like your soul reaver regiments (vampire knights) and it's often your MVP unit, instead of spamming two of them, maybe you could get similar results with only one regiment and one troop, or one regiment, and one regiment of cheaper but still effective revenant cavalry. Or maybe even a different type of fast units like werewolves or wraith, it depends on your play style.
In short, don't simply spam your best units, try to keep some balance so that you don't end sending a 350+ points dragon hero to do the work that a 80 point mounted hero could do.
It's usually easy to find a decent job for one dragon, but as you start taking several it's becoming harder to use them without wasting part of your potential, and it won't matter in the end that you completly crushed a poor 100 point regiment with your 700 points of dragons if in the same time the 1900 points left in the rest of your opponent army crushed your 1300 points thanks to their own relative advantage.
It's possible to make armies with a relative low number of drops work, but it take a good knowledge of the game and a lot of finesse and your margin of error will often be rather small.