I think there are tiers of this. Its true that balance is more difficult at higher levels. That said, even if you take a regular old "balanced" party of the core roles (fighter, wizard, cleric, rogue) and go into high level fights you can quickly see that the standard CR encounters don't cut the mustard. The monsters generally just don't do enough damage, and can't take enough damage, to remain a threat. Further, part of the problem is that abilities just scale up...but to challenge high level parties you need more "epic" abilities to really do the challenge justice. Dragons are probably the worst offender here. High CR dragons are just the same as lower dragons but with higher numbers. They don't get anything new and unique to deal with the power of high level threats.
One example from 4e that I used for high level games.
1) Low levels: I used the standard "ongoing damage". Example: Take 10 fire damage. Make a save. If you fail, take 10 fire damage again next round. Rinse and repeat. This was a normal 4e ability.
2) Mid Levels: I introduced "escalating damage". Example: Take 10 fire damage. Make a save. If you fail, take 20 fire damage next round. The round after that, take 30.
3) High level: I introduced "crisis damage". Example: Take 10 fire damage. Make a save. If you fail, IMMEDIATELY take 20 fire damage. Continue making saves until you pass or your dead.
Its not just higher damage, but the frequency increased to match the power of high level opponents. And let me tell you, that crisis damage scared my high level players!
This also has a lot to do with GM tactics. I see a lot of people complain about the Dragon CR being too high but I have also seen people walking the dragon around on the ground vs a bunch of melee Player characters. It's flying creature that has a massive AOE breath attack. If the dragon is getting the crap beat out of it it is defiantly smart enough to go airborne and rain down from above. Also, if a dragon can fly it has at least 10ft for both the bite and tail attack, and Ancient dragons of 15ft so they can fly above enemies with a 5ft melee range and attack them without being attacked back. With the 15ft of ancient dragons unless it is fighting all range and casters it SHOULD be doing just that out of reach of pole-arms. Even if that is one target not doing damage it makes the fight harder and that player has to work to become useful. Also, add to that players with magic swords and melee abilities having to use mundane crossbows for a lot less damage if they are unprepared.
If your party is all arcane archers and spell casters with long range spells you also need to remember that the Dragon has a power full AoE and moves 60ft to 80ft in a turn where players generally move about 30ft. That mobility is important to a dragon. Your casters group up then they get devastating AoE breath on character that generally have less health than there melee counter parts remembering that the dragons breath changes range from 30 -> 60 -> 90 means with dragons mobility and lack of attacks of opportunity in the air they are almost always going to have a good shot for their breath attack just worse if they group together. If they spread apart 35/65/95 ft then they are reducing their ranged attack attack options due to spell range limitations. Even spells with 120ft range can have problems against the ancient dragons because if your 95ft on the other side of your ally from the dragon and it is 15 ft from him your range has to be in a 10ft ring any clever movement or obstacles with line of sight at that range are going to be a problem since at that range the movement to over come angles of cover is too great for the player.
This requires the GM to put the dragon in field of battle large enough for a creature of this size to fight as it is designed instead of on the ground in the dragons broom closet like I usually see. Of course if the dragon is "trapped" or cornered on favorable terrain then that was the players taking head of the danger and one of the dragons greatest strengths and trying to limit it.
That is not a reflection of the dragons CR rating but the players tactics to deal with such a creature at that point.
Some people will say that's a cheep
tactic... but I would argue that's exactly the
tactic a very smart very old dragons fighting unfair fights of 6 to 1 would, should, and likely have used in order to live to be that old and that since of helplessness and despair that melee characters get when fighting dragons is part of the terror of dragons. It's also something players should expect from one of the most notorious airborne enemies in the game of Dungeons and Dragons.
If your playing a creature against one of its greatest strengths then claiming its under powered for its CR ... its not the designers fault the CR system is not working out for you. I have seen battle after battle of Dragons sitting still and taking a beating from 6 sides with multiple 5ft melee attackers in the group and then called an easy kill its ridiculous.
If you saw the critical roll fight with the ancient black dragon (
Umbrasyl) you saw
more of the the sitting dragon, then the fight with the white dragon (
Vorugal) was done much better and it was a bit of a terror. In those fights I think Matt has some landing due to personalty of the dragons though its not the best example because he defiantly modified them for his players and to make them unique, I do think it the most public example and does illustrated big differences in difficulty from fighting dragons in tight spaces and open spaces. Which are the result of GM and/or player tactics not something that could be part of selecting their CR.
As a side note if a GM wanted to put players against a dragon early he could put players in a tight room and let them beat on it and if he wanted to make it harder later he could have them fight the same dragon out side in a ravine with oh 15ft foot walls that would help break line of sight but still leave players on the edges in dragon melee range when it was landed on the sides as well as forcing them to group up.