I wana hear what eric has to say on this subject
My Meager Insights:
The more cube I've been playing (across various types) the more I've been confirming the starcraft rule. Being able to spend your mana productively every turn is so important. I feel like this is one of the biggest issues with midranged and control decks in most classic cubes, the awkward colourless mana, the shallow pool of draw fixing, curves with weird saturations, overabundant resource denial and how inconsistently they are able to get beyond the point of merely playing one spell (relevant or no) per turn.
These cubes really rely on doom blades and bolts because their control and midranged cards tend to cost 4-6 mana and path or an elf are really the only way they are ever going to be moving forward while casting deep anlysis/ etc. I've also found in these cubes that your 3cc spells take up a full turn waaaaay too much of the time. I'm not sure why this is the case. Maybe it's fixing issues and being forced into 3 colours (overabundant colourless mana) or maybe it's wastelands and braids etc. Maybe it's because it's hard to find a lot of 1 and 2cc spells that are usually relevant on any given turn in a draft.
I found from playing Eric's cube and various peasant cubes that being able to spend mana consistently and productively is double important. Things like snapcaster mage become more awkward than you'd think because you feel a lot of trepidation about the use of it, as opposed to just say, compulsive researching into a better hand. Things with activated abilities like DRS, Ooze, Any solid bestow or monstrosity creature, equipment or flashback spells feel so much more relevant to me. I feel like this is because I rely waaaay less on trying to make advantageous trades in these cubes and I feel like I need to be proactive where ever I can. Any silly combo loop I can put together will probably be powerful and I feel like I have way less control over in what order I play my spells unless my curve is super super low. If I have the Oring in hand, and he plays a threat, I probably want to kill the threat with the oring, unless I have a relevant blocker in my hand to play, saving things for later is way less important. A lot of this has to do with the compressed power level. I don't need the oring to kill a jitte here. I need it to not die to 2-3 drops.
The one exception to that rule is how often you encounter voltron decks in peasant formats, where if you get even the faintest wiff of one, you want to save your removal to do something tricky.
So yeah, mana sinks, and having a lower curve, even for control and midranged, is almost as good as having access to more card advantage a lot of the time. Decks have way less oppertunity to have interesting turns when your cards cost more or you don't have ways to spend small amounts of mana outside of your hand. You get into slugfest magic, and that isn't the loopy interactive good time I am looking for. It's odd the way these flattened curve out cubes have encouraged me to play every combo I find but, I end up discovering way fewer of them or fewer opportunities to create interesting interactions between me an my opponent. Paradoxical.