Most 40K units fit under three basic types: slow assault, fast assault and shooting. (There are fast shooters, but they are expensive. You seldom see one trick pony forces build around fast shooting.) Assuming people build one trick forces of those three types, the shooty force can kill the bulk of a slow assault force before they reach close combat. The fast assault force can get to the shooty force before they take critical casualties. The slow assault force will outnumber the fast assault force, so it doesn't matter how quickly the fast army gets to the slow. This is a classic three way paper-scissor-rock balance.
This balance can be broken by too much or too little terrain. Block enough line of sights, and a shooty force is in trouble against anyone. Open up the field too much and give the shooty army first turn, and even a fast army may find itself critically limping. I have seen stores where rhino rush dominant cliques declared 'the more terrain the better the game,' thus making sure that the dominant clique won victories over other styles of armies. I also played occasionally with a group of shooters who learned the game playing on their living room floor, little terrain, with armies starting 48 inches from one another. They too used peer pressure to force terrain favorable to their preferred style.
As a player who prefers a balanced army, I find it important that this triple balance not be broken, that the terrain remain moderate and variable. If various styles of focused armies are winning against one group of focused rivals, but losing to another, my balanced armies can do well against any opponent in that terrain.
Thus, within reason, a one trick pony player wants to field a force that gives him an advantage against one foe, but puts him in trouble against another, I don't object. I'm generally 50% long range shooting, 50% fast swords. No matter what opponent I face, no matter what table I'm on, half my army will love it. The hot half will just have to to take up some slack for the other half. It's not always easy, but I'm generally in the game.
So, yes, if you build an extreme force, you will be claiming easy victories over some opponents, and shooting yourself in the foot against others. The fast, slow shooty balance is just one aspect of it. The quality / quantity trade off is another basic trade off. I've seen some stores where marines armies are designed exclusively to fight other marine armies, and the thought is that the more high AP armor defeating fancy toys you have, the better. As that thought is pursued, the Marine armies get smaller and smaller as they become fancier and more expensive. You can end up with tiny armies without enough weapons to stop a hoard. If someone comes in with Bug, Guard or Ork forces built to maximize numbers, they get into trouble.
So any time one goes for an extreme, expect some easy match ups and some ugly match ups. Sure, depend too much on terminator armor, one ought to get nervous facing Eldar sniper rifles or starcannon. Some balances are broad and abstract, such as fast assault / slow assault / shooty or quality / quantity. Others are unique to particular forces. Build an extreme army, spending all your points on one thing, some army or another is going to have just the right stuff to take that one thing away from you.
Which is why I say extreme armies are high risk high reward. They will give you some easy victories. They will occasionally fall apart big time.
Me, I like balance. I'll take two legs of the triple balance, and lean towards numbers rather than quality. I won't win every time, and I seldom win without sweating some, but I'm almost always in the game.
Last game I played was against Ultramarines. He focused primarily on middle ranged shooting. Vindicator, Vindicator, rocket devastators, terminators, terminators, dreadnought, speeder, speeder, speeder, and about five 5 man troop choices with a lascannon or plasma cannon.
I had obliterators, lascannon havocs, heavy bolter havocs, 2 full sized sword and pistols undivided with power claws, 3 mixed sized boltgun units, 3 8 man fury squads, and a winged demon princess.
He deployed towards the rear, assuming that I'd have to come to him, given that I had spent lots of points in close combat stuff. I deployed back as well. I had three focused long range killing squads, while he had one. I thought I'd do OK in a long range shooting duel... but I infiltrated 6 boltguns and my winged demon princess way out on my left flank.
My lascannon took out the first vindicator before it got range. My flanking force had a few nervous moments looking down the nose of the other vindicator, but summoned the first squad of furies, who got into the face of the rocket squad. Suddenly, I had 3 long range heavy squads. He had five individual weapons in five squads. The long range advantage was mine.
Meanwhile, back on the left flank, summon summon. Happy scatter. I got a squad of furies to the rear of the other vindicator. Boom. Another fury squad engaged the first of the undersized troop units. The last of his rockets died to the original fury squad. Two land-speeders tried to duel my main fire-base, and lost. Badly.
On my left flank, I had three squads of furies and a fast demon princess. In his center, he had a dreadnought and terminators. He had an ugly choice. If he kept his heavy units back in his center, he could prevent my fast wing from sweeping up his tiny troop units... but they would have to stand in front of three long range heavy units in order to do so. Instead, he sent his center towards my lines, charging my heavy support with his heavy infantry. Only the dreadnought made it, killing the obliterators with his assault cannon, only to fall to three power claws of the sword and pistol marines, who had stayed home to screen the heavy support choices. The fast wing was left free to sweep the Ultra's rear. Ever see what 16 furies and a demon princess can do to 5 marines? One squad a turn, then on to the next...
I have a tradition. The star unit in a given battle gets to open the keg. Who was the star? I gave the keg to the six boltgun marines on my flank with the demon princess, who walked into the face of a vindicator in order to summon the furies. At the end of the game there were two of them left... dancing on the hull of a speeder they had taken down with bolter fire.
Anyway... He chose to be strong at one thing. In his case, it was middle range shooting. I was superior at two things. Long range shooting and fast close combat. I had to find a way to utilize both my strengths while avoiding his. I infiltrated and summoned on the left flank, out of range of most of his squads. My massive long range firepower dominated in the center. A fairly minor exchange of skirmishing fire on the right flank was not decisive. I had the options given by variety, range and speed. He had to stand and take it, because moving at long range meant he couldn't shoot.
It doesn't always work. If that second Vindicator had really tagged my flanking force early, it would have fallen apart.
But somebody has to open the keg.
And when playing paper scissor rock, it is really neat to have both a scissor and a rock... though someone with a really big rock can still be a pain.