In RIFTS, the charts weren't on the pages for each class. They weren't anywhere near them. They were impossible to find, level after level. This is because Kevin Simbieda is insane.
Obviously that wouldn't be the case in D&D5, and maybe confusion was the wrong term. There are various issues caused by differing XP curves:
1.) New/young players getting confused about how much XP they need for the next level, when their fellows are all leveling at different rates. This may seem dumb, but I know this confused and frustrated me when I was a kid playing 2e.
2.) Breakdown of meaning in what a "level" is. If a wizard hits level 3 at 5000 XP and a fighter hits level 3 at 2500 XP because the wizard is twice as powerful, why are they both being called "level 3?" This means that levels don't correlate with power OR amount of XP. If the wizard is more powerful at level 3 than the fighter, they're not at the same level.
3.) Breakdown in time between rewards. This may be a feature-not-bug to some, arguing that wizards should have to wait longer to get to the good stuff, but it seems to me that playing a class who has no new powers, features, bonuses, abilities for twice as long as your friend could be frustrating and become boring.
4.) Breakdown in maximum power caps/character usability time. If a wizard levels at more XP than a fighter, because he gets more power per level, then a wizard effectively has more overall length in-campaign AND more overall power than a fighter. A wizard's player can use that character for much longer (because he'll take much longer to reach level 20 or 30) and when he hits level 20 (or 30) he'll be way more powerful than the fighter who hit level 20 6 months ago, especially if that fighter's player started a new character in the meantime. Or should the fighter's player just play a static, unchanging character until the wizard maxes out?