Except that I don't think there is any such thing as a "completely balanced" system.
I agree. There's no such thing as completely balanced. There is a difference between "Roll a saving throw, if you don't get a 20, you are dead" and "Roll a saving throw, if you get over 14 you take 15 damage, otherwise you take 30."
It's a matter of degrees.
As I said, I don't think that a perfectly balanced system. For me, part of the fun of being a GM is seeing how a monster's abilities match up against the PCs, how it can challenge them, how it plays to their strengths and weaknesses.
It's all math, however. Often the math is lost when you are looking at a monster that is so complicated you can't calculate it easily in your head anymore. This is what tended to happen when I ran high level games in 3.5.
I'd look at a monster and it'd have 10 different spell-like abilities, 5 different resistances, a weakness or two. 4 special abilities, and a bunch of stats that ranged from 3 to 50. Did the fact that it had +32 to hit mean it was going to win or not? It would certainly hit 5 out of 6 of the party on a 2. But one of the fighters had an AC of 42. Plus, the monster only does 1d8+8 points of damage per hit...but it gets 4 attacks per round. It's weakness is also something the party wizard might use against it. But it's possible that the wizard gets lost in attempting to hit it with the things its resistant to. Its special ability might stop the fighter from engaging in melee...but it only has a 25% chance of succeeding. It also has another attack that could kill the entire party if they don't succeed on a 3 on their saving throws.
And when you have to consider all of those factors then its effectiveness against the party is pretty much a flip of a coin anyways. Purely random whether it wins or loses based on a couple of die rolls(possibly as little as 1 die roll). On the other hand, if you simplify the math to the point where it is a lot more predictable, you can fairly accurately say how likely a monster is to kill someone while still leaving things to chance so it isn't 100% certain.
I don't expect everyone to have similar tastes, but I, personally enjoy reading through monster/NPC statblocks to see how they compare to my PCs. I never grab one at random and run with it. YMMV.
That's fine. There's certainly nothing wrong with enjoying that. I love going through stat blocks when I have time as well. I just didn't like the unpredictability of it all. Sometimes it was fun to look at a monster and think, "Ha, it has enough fire resistance that the wizard in my party who ONLY uses fire spells will be useless against it. And its AC is too high for the fighter to hit. And it's only CR 14. I could use it against my party and it would probably wipe them out, even though they are level 16. And I could claim I even used a weak monster against them if they complain that I tried to wipe them out."
And sometimes I'd say, "I don't have time to look through this in detail, but I know they are going to the elemental plane of fire, the flavor text on this creature says its native to that plane and it's CR 14, so it should be pretty weak. It'll give them a taste of the plane...and I can use 4 of them, that'll only be EL 18. Perfect." And then I'd easily wipe out the party.