You're overthinking it. This is probably the most simple to introduce mechanic for cube in a long time, honestly.
Compare to.. oh, I don't know, any of the other shit we ask players to keep track of? Creature type line? The graveyard as a near-constant concern (for most of our formats)? Duplicates? Level up counters in an environment that also supplies +1/+1 counters? Indicating
Warden of the First Tree and his bizarre pseudo-level up ability? We ask a lot more of our players every game than energy.
The comparison between the elegance of Energy and the absolute trainwreck shitshow that is the sixth colour of Magic
is a bad one.
requires difficult mana base formulations, and manabase is already part of the hurdle that most players struggle with the worst. Adding a sixth colour of Magic makes the hardest part of deckbuilding significantly harder, harder than most seasoned drafters here probably truly appreciate.
Meanwhile, Energy is largely self-sufficient. Cards generate their own Energy so far in some fashion or another, and have such obvious and natural synergies when you put them side-by-side; "Oh, I can save my energy from X card to expend on Y card. Neat!" Going back to a concept I use in my own cube development, which I've referenced here, Energy is a package we can easily include, as it offers lines and signs simultaneously across all cards. It's marvelously inclusive and easy to understand. I'll likely be giving each player an "Energy card" to stack energy counters on, and that'll be the end of it. This is the best new idea in Magic in a long time, and I guarantee you, players are going to learn it fast and love the thrill of trying to unlock its full potential.