The big problem I have with adventures that need the PCs to level up is that it almost hard-forces a certain speed of advancement even if that's not what the DM wants. For example, one of the tricks I use for making a campaign last longer is to greatly slow down the advancement rate, which runs into trouble if I want to use any of the published modules...unless of course I just accept the fact that the early parts of that adventure might be a cakewalk for the PCs and the latter parts might be above their heads.
I could have written the same thing.
I am probably very old-school now when it comes to a game's difficulty curve (not just a RPG), and it would make more sense to me that the game gets more difficult as you progress. So I actually do prefer to accept a cakewalk-to-challenge-to-nightmare progression, over the default of published modules.
I also push back against milestone levelling in favour of individual xp so as to allow characters to come and go as the players desire; and to not have it that characters get xp while they're dead or otherwise not adventuring for a while; and because I'm a bit chaotic and want to occasionally see elements arise in the game that bestow or remove xp or levels to lucky or unlucky individual characters (e.g. Deck of Many Things, level drain effects, magic surges, etc.).
I've done all these in the past. In long-running 3ed campaigns, I've always given XPs only to present characters, not absent ones. Different XP per characters in the same session is something I did only at the beginning but then saw only problems, and moved to always dividing XP equally, but only among present characters. And yes, I'm a fan of having level drain in the game as well, even though my favourite is actually ability score drain.
OTOH, it's not wrong to even completely throw the idea of XP as a reward out of the window, and simply
have the characters be the appropriate level for the current adventure, if the group mainly plays adventures serially, rather than concurrently or sandbox-y. I know that XPs are traditionally treated as a
reward, but it doesn't have to be the same for everyone, there's lots of rewards inherent in playing the game itself. It's like videogames after all, some people play for high scores and do not understand why someone else would play a game without a score, while others do not need a score system at all and are only interested in going as far as they can.