"Also, I would like to add a card to that poll if possible: Quantum Pillars/Towers. Those things desperately need to be nerfed in a big way. "
NOOOO!!!!
Indeed, Quantum towers are widely used, but not because they are powerful. They are not. They generate 1/4 of quanta per turn per element on average as opposed to regular tower generation of a whole quantum. That means you can play every card in your hands four times slower than in a mono deck. Playing Rainbow with QT is very very slow to start, and unless you manage to distribute all you card equally in all 12 elements, whichever element you draw multiple cards from, you won't have enough quanta for those. QT is also random : it increased the factor of luck significantly, providing more luck related losses for you.
QT are so slow at start that without either Supernovas and Sundials(stall for more quanta for free), rainbow decks would be probably useless.
Most games I lose with a supernova based rainbow are lost due to not having enough quanta to play my cards...you don't always draw those supernovas, and QT is too slow.
The reason why QT is played is not its power level.
QT is played because there is no alternative.
Each element has only 5-6 mono card, and another 5-6 cards that need a second element to be useful (like Dune Scorpion, Forest Spirit, Lava Golem, Lycantrope, etc). This is very limited. If people need certain abilities for their deck or want to use some combos, chances are, they'll be from different elements, and they need to use a minimum of 4-5 elements to build a good deck, and at that point, using QT is a better alternative than towers/pendulums : it can provide quanta from the remaining 8 elements...and since it doesn't work well with just 4-5 elements, you'll include more.