I'm torn between two sides here - one part of me sides with satisfaction, and another sides with designer's intent.
On the one hand; having stuff that requires lengthy games to access (biggest examples being Pulsefire Ezreal's/Elementalist Lux's transformations) is pretty bad when you take them into gamemodes that have shorter match length which don't accommodate the later-game stuff well.
On the other hand...Elementalist Lux was designed for players to do their work, change to a new form of their choice, and then combine it with other forms to make a new one - no changing one's mind, and no going back.
Each Ultimate skin has their own unique schtick, in this regard - Pulsefire Ezreal gains armor with his rank in Trueshot Barrage, Spirit Guard Udyr gains a primal aspect in the abilities he reaches max rank in, DJ Sona can cycle through her tracks at-will, Elementalist Lux goes down given roads for her transformations, and Gun Goddess Miss Fortune can load up a new suit in the safety of her base.
And all of these limitations (or lack thereof, in some cases) provide some novelty value to the skins. They're all fundamentally similar, but vastly different in how they function.
On top of that (maintaining designer intent and novelty value), asking Riot to push even one Ultimate skin to be similar in function to another effectively sets that Ultimate skin to be a baseline for functionality - an expectation that would need to be met universally across each Ultimate skin.
PF Ezreal would have to be given the ability to cycle through his armor upgrade states, SG Udyr the ability to cycle between mastered/unmastered at will, and GG MF would have to be allowed to switch out her get-ups outside of the base and armory. And any future Ultimate skins will likewise be held to that standard - they have to be able to cycle through their forms like DJ Sona, because that just becomes the rule.
That all said...It would be inane for me to claim I'm of a split mind on this without having equal reinforcement for the player satisfaction side as I have designer's intent - and, I do, in the form of a simple proposal;
Have Riot tweak Elementalist Lux's power gauge to accommodate faster gamemodes. It's a short, sweet, and simple solution that satisfies both sides, right?