Well it is more complex when everything is relative. There is also the complication of how I think it was intended to be used and how it was actually implemented. The idea is that every 5 levels there is a 2x jump in power. This is roughly expressed by the change from minion - standard - elite - solo. So a level 8 Ogre Savage, would also work as level 13 Ogre Savage Minion, or a level 3 Ogre Savage Elite for a PC at those levels. However, the Ogre Savage is a brute (means a lot more HP), and there is no minion brute (so that doesn't scale correctly). In addition a problem that 4e had was that they inflated all of the monster levels when they stretched everything to 30 levels. This leaves you with strange situations (like the ogre) when you try to scale things up and down.
For example: The balor in 4e is a level 27 elite, an "epic" threat. However, balors are not "epic" threats and should have been levl 18 or so (like every other edition). If everything is about 2/3 the level it was at in the MM the whole system works better (and you need minion brutes too)
Not sure I agree with this analysis. First of all the delta between minion/standard/elite/solo is actually 8/4/5, so a level 1 solo has the same XP value as a level 6 elite, a level 10 standard, and a level 18 minion. So lets think about the level 3 solo white dragon. It has 200 hit points and an AC of 18. If you converted it to a level 8 elite it would have the same 200 hit points, an an ac of roughly 26 (the white dragon is pretty far over the standard DMG value for AC BTW, a normal level 3 brute is only AC 15). As a level 12 standard it would have 140 hit points, and AC 27. Obviously as a level 20 minion it would have 1 hit point, but its AC would presumably be around 35.
So, lets analyze this in terms of how we might use these different creatures. At level 1 the young white dragon is a terrifying single opponent which is hard for a party to beat as a single opponent. This is how it would be used, or possibly with some helpers against a party of maybe up to 5th level. Past that it wouldn't hit well and its defenses would be too weak. 200 hit points wouldn't even last 2 rounds against level 5 strikers (maybe not one round). However the level 8 elite version would be an interesting addition to an encounter, possibly representing the reprise of a dragon driven off in an earlier low level adventure. Its 200 hit points would make it modestly durable at its new AC. With reworked elite-appropriate damage output it would be pretty similar to the solo, just with more usable numbers.
As a level 12 standard it would simply be a front ranker in some boss monster's entourage, perhaps serving some more powerful dragons or something as a scout or something similar. At this point 140 hit points is barely enough to survive most strikers one round encounter power damage output, but with a team of monsters it could contribute. If you used the original level 1 solo stats it would be pretty awkward. The damage might be better, but it would be a tedious process of trying to get a hit.
As a minion the thing is going to obviously suck at level 20, but it is now just one of a SWARM of young white dragons arising as part of a paragon capstone encounter or something similar. It might get off an attack, but frankly even if you gave it 140, or even 200, hit points it would hardly matter, and with the AC of a level 1 creature it would be just as easy to splatter as a level 20 minion, but a lot harder for the DM to run.
Sure, you can argue that the AC of the young white dragon's scales 'means nothing', but AC isn't an in-game concept, and it doesn't even really map to any single specific in-game thing. That was true in AD&D as much as in 4e. There was always a sort of trade between AC and HP in terms of how tough things were.
As for AD&D Balors, if they weren't 'EPIC' nothing was! A type VI demon has an average of 44, and a maximum of 72 hit points. They have strong psionics, require magic weapons to hit them, are 75% magic resistant (and then save as a 10 HD monster). They fly at 15", are AC -2, and can quite easily pump out anywhere up to 28 points of damage with a whip/immolate attack (that is enough to toast your average name-level wizard). Of course this is not really TOO fearsome, a high-level fighter can deal with these stats no problem (level 12 fighter has average of 76 hit points and can probably dish out as much as 20+ damage on a hit and gets up to 3 attacks, vs AC -2 he'll probably hit 75% of the time too).
Problem is, this monstrosity has a LOT of magic! First problem is 15" fly speed, so the fighter is SOL most likely. Next is teleport without error (no limits on use, AD&D doesn't really have 'action economy' so its unclear if they can kite with this). There are the symbols it can use (fear, discord, sleep, or stunning), suggestion, and a 600lb telekinesis (no range is given, I'd go by the spell, this can be NASTY). They can cause fear, and there's about 10 other less instantly lethal but highly useful abilities. All of these are ABILITIES, not spells, so there's no interruptions or limits on when or how they can be used, they simply work instantly. Plus they can gate...
So yeah, a Balor is pretty epic. I mean, lets say that 1e tops out at level 20 (it actually doesn't say you can advance beyond that, though its implied, all the charts end at 20). That's the equivalent of level 30 in 4e. A 1e balor is AT LEAST on a par with level 12 PCs, and is certainly the equal of a single PC of up to level 15, making it roughly equivalent of a 4e level 23 standard monster. I'd pretty much argue that its a top-tier creature, as Juiblex and Yeenoghu, at 88 and 100 hit points, are not THAT much stronger, and those are certainly 'epic' monsters. Truthfully I think creatures of this ilk kind of just work a bit differently in AD&D, so once you hit the sort of 'epic levels' (beyond name level) its hard to exactly set equivalences. Fighting these nasties is tough anyway because its a lot easier to die in AD&D from some small bad luck.