Unfortunately, those few people are wrong. There's no logic to that argument and denying that some units and armies are far more powerful than others is plain dumb.
In an attempt to put an end to these retarded arguments, I'm going to do my best at describing what makes something overpowered, or more powerful than something else.
|Remember 5th edition when Necrons were balanced? Oh well...|
Simply put, something that's overpowered is something that is too effective, efficient, can perfom too many roles too well or has too few weaknesses for its point cost.
But let's look at the specific arguments often put forth against the notion that anything could ever be OP.
"It all comes down to the skill of the player"
To be fair, this has some credibility to it, as more often than not the more skilled/experienced player will win, but that's not what I want to get at.
My issue with this point is that some units do not require skill of any kind to use, because they're good at everything. But it's not just that they're good at everything, it's that they're too good at everything. For example, standard Space Marines are pretty good at doing whatever you need them to do, but they're not the best at anything and, to be quite jeff, they're pretty bad in combat.
Instead, think of standard Grey Knights, especially back in 5th edition. They're like Space Marines, but for a small points increase they become monsters in combat (and can annihilate monsters in combat at that) and they even benefit from superior firepower. They're too good at everything, and in comparison to Space Marines they require less skill to win with, and nothing should require less skill than Space Marines.
"It's all about the luck of the dice on the day - Warhammer is all about luck"
I hate this. It's the stupidest argument of all time. This is what people say when they go into "goddidit" mode. When they want to hold onto their belief that their army isn't at all overpowered and that they're really a good player, they deny everything and just tell you that "thedicedidit".
Unfortunately for you, my terrible gaming friend, it wasn't entirely the fault of the dice that made your unit beat face. See, while everyone has to roll dice and hope they don't roll any 1's, some units have a better chance than others at doing what they want to do, and in some cases this chance is just too good for the cost of the unit.
You want an example? Just look to the Hell Pit Abomination. That thing is 100% random, so unlike just about everything else in Warhammer, it is literally decided by how your dice roll. However, with 3D6" movement and his stats, as well as his special attack types and Impact Hits, the Hell Pit is insane. It's literally a "no skill involved, just throw dice at it and win" unit.
You want another example? Alright, let's get a little more abstract, 40k style this time. Unit X and unit Y. Both cost 10 points a model. Neither of them have any guns, because they're both purely designed for combat. The difference is unit X is WS5 while Y is WS4. There are no other differences. Are you willing to sit there and tell me that unit X is not overpowered? I mean, there's always the chance that X rolls all 1's and 2's while Y just 6's everything. Yep, definitely a possibility. But the fact is that X is far more likely to Hit than Y, and that's what actually matters.
It's about the chances of the units to do something, and some units have odds that are just far too good for something of that cost, whether it's because of a huge number of shots/attacks or because of a stat that's just a little too high, some units are too powerful because their chances are just far too good.
"It's about both - skill and luck"
Let's look to Skaven for an example, more specifically, Skavenslaves. Alright, so they're WS2, BS2, S3, T3 and have no armour. How on earth could they be OP? Well, we've got to factor in a couple of other things first. First of all, a general is mandatory in just about every army, and considering how important magic is and how incredible the Dreaded 13th is, just about every Skaven player will be running a Grey Seer. Second of all, BSB's are generally considered a must-have, especially in an army as cowardly as Skaven. So, Slaves not taken into account, these two things would be taken almost all the time anyway.
Now what that's got to do with Slaves is this: Take 50 Slaves in a unit, 5 wide and 10 deep, and keep them in range of your BSB and General and they're a leadership 10 with rerolls tar pit at 2 points per model unit, as well as being Steadfast in just about every situation for a very long time. Then, add onto that the fact that I can sit back and fire my Warp Lightning Cannons into that combat for as long as I want while my opponent can't do anything outside the combat to help his guys win, and then add onto that the fact that other units don't panic when Slaves explode, and you've got a very, very good unit for only 2 points per guy.
This unit requires no skill to use. You simply get it to tar pit whoever you want and keep your general and BSB in range of them. It's so simple that I can do it. It also has a very, very small chance of running away, so it's a 2 point-per-model tar pit with no weaknesses in its role. In fact, it's better than a whole lot of other units designed for the task that cost way more points than it does. That's OP.
So I don't know if I demonstrated my point very well or not, because I had a good amount of trouble trying to set this out nicely for you all, but hopefully you catch my drift.
The main thing is, some things are more powerful than others. Some things are too powerful. It's just a fact. Deal with it.