Certain armies are built to be competitive - and it's those who have units which have good stats, and good rules which are constantly in effect.
For example - Empire Greatswords - they have 1 ASL S5 Attack each, with T3 and a 4+ Armour Save. Not very good - the standard S4 opponent's attacks will hit on a 4+, wound on a 3+, and not be saved 2/3 times - meaning that a 22% chance of dieing to every attack coming towards you (and when you have often multiple attacks or higher strength attacks at higher initiative against you), they appear to be at best mediocre.
In 7th Edition, that was why they were a poor choice. Expensive, and they had two special rules which made them useful - Detachments (Handgunners in particular), and Chargers Strike First.
They still have detachments, but they lost Chargers Strike First. However, in response to that loss, they now have Step Up, and Supporting Attacks. They each only have one attack, so no points are being wasted on attacks which aren't used, and Step Up means that ASF isn't a problem - especially thanks to stubborn, meaning that they can even go into Horde Formation if they so wish.
The last update which makes Greatswords such an effective unit in this edition is that they no longer take up a Special "Slot". In the past, being limited to 4 Specials at 2000pts meant that it was often a toss up between them and another unit of Cannon, and Cannon being more useful and a lot cheaper (costs about the same as 8 Greatswords), was often more widely used.
Now however, you can have a unit of Greatswords, 3 Cannons, 3 Mortars, 3 Pistoliers, and still have points spare from your Special Limit.
The greatest thing, though, is the ability to be consistent. If I can have a consistently mediocre unit, or an occasionally powerful yet occasionally useless (but not game winning) unit/tactic etc, I'd take the mediocre one.
Case in Point - Chaos Trolls - they have Stupidity, and Ld4, yet are among the hardest Monstrous Infantry in the game, with a Magic Attack to deal with Ethereals, Multiple High Strength Attacks, a 4+ Ward Save, and an extremely useful Roll on the EotG Tables should they pass enough Ward Saves. (Well, it's Regen, but large numbers of Flaming Attacks aren't that often found aside from Tzeentch Daemons and Spells). I wouldn't take these aside from 2 Things - A Battle Standard Bearer (Reroll Failed Stupidity), and Throgg (Ld8 Trolls). With Step Up and Supporting attacks, along with Mutant Regeneration, a unit of elite infantry can charge/recieve a charge from Trolls, cause some wounds, only to have at least 2 Ward Saves Passed, they then become Stronger (+1 Attack/Strength mainly), and they then proceed to break apart a unit.
Other units are ones which are flexible - Dragon Ogres - can take both Great Weapons AND additional Hand Weapons - with Monstrous Infantry Rules, they have Supporting Attacks, and can take a few hits (thanks to a 4+ Save and T4), yet can retaliate to armour or massed infantry with their multiple attacks, and stand a good chance of winning through. If not, a unit of 6-10, despite being extremely expensive, has 24-40 Wounds, more than a Chaos Warrior Unit, more than many units out there, and often more dangerous as well than units any larger in size.
So - in short - Static Rules, not random rules. Of course, stats come into it, but reliability over randomisation is usually better. 4 S5 Attacks is better than D6 S6 Attacks.