I applaud your ability to run high level campaigns, and in fact I even envy you.
That said, I think the Adventure Path we ran worked pretty well, even at high levels. (Well, at least I think Banewarrens and Shackled City were pretty successful. Age of Worms was a disaster for us, due to continual high lethality, and the DM seemingly incapable of using the adventure path correctly.)
Problems begin if someone has (or at least: If I have) to create a long-running campaign.
1) Ability to create a continous, compelling storyline to entertain both the players and myself. I had some high level ideas for D&D, Dragonstar and Arcana Evolved, but somehow I never managed to see it through the end. I think it might be a multitude of other factors, but I think one problem of me is that I try to have an idea where a campaign will lead up to, but somehow don't manage it well to convey this to the players. It's not that they are disrupting my railroad attempts
, it's more like what I have in my mind doesn't come out as good in real play. Though I am not seeing this change much with a different game system...
2) Preperation for high levels. Creating NPCs takes a lot of time, at least it used to, when I was still trying to do it all "by the books", optimizing skill points to model my idea of the NPC perfectly to the game rules (going all simulationist), even though I knew the PCs would see the NPC only once, during a violent meeting (some refer to it as combat). Stupid me.
3) General gameplay at high levels. I don't enjoy the "instant death" effects that come as Save or Dice or just massive damage in surprise rounds. I just don't like the Save or Die/Ressourection Cycle at higher levels. As a DM, it's not so bad. I can pull the punches if it appears as if the PCs are close to death. But as a player, I don't have this control.
The Death & Life Cycle ensures that my hard earned XP and GP are for nothing, and the likelyhood of my next death has just increased, but its meaning has been diminished greatly.
I think it is a good strategy to run published modules and just adjust them as you see fit. It is less work, and usually, the important stuff is taking care of you.