I did look up Moonflair Spellblade, and all of the stats seem pretty good, but the tenacity part might be too much if on CC mages, I'll agree there, but some fine tuning can certainly make it balanced.
Moonflair just seems like a very defensive item, which we need more of if we fall behind in lane, or need something in case the other team has an equal share of AD and AP, somewhat of a 4th or 3rd item, depending on how the game goes. Zz'Rot and Guardian Angel offer the same amount of armor and magic resist, but I wouldn't dare buy Zz'Rot on Annie, I'd lose out on offensive capabilities, not to mention Zz'Rot is a niche item for high health tanks if they want to split push but also be with the team (the voidlings deal more damage based on your health).
Yeah, I'm not too sure about the tenacity vendetta either, when the champion pool (and most of the champions played nowadays) all have slows or something that would be nice to build tenacity against. Probably because it made balancing of certain characters harder if they had to put into account that there might be some tenacity thrown into their build with some items (Zephyr and Mercury Treads).
Imagine having Moonflair, Zephyr, and Mercury Treads on Irelia. She would get an exponential amount of tenacity (items and passive), and practically ignore any CC applied to her. Now, that would make her really dominant because she can never be slowed, stunned, or rooted, and those are the three main CC abilities that make ganks and kills the most reliable.
And if there was an ungankable champion being dominant thanks to items making other champions' lives easier in other lanes, then everyone would be unhappy that one champion has to ruin the fun for everyone else.
Lissandra with tenacity? Imagine her with a Rylais. Good luck getting away from her.
I think the main problem is that there are items that give AP and health, but there are very minuscule amounts of items that give AP and defenses like armor and magic resist, which we need more of.
Abyssal gives AP and MR, Zeke's gives small amount of AP and armor, Zhonya's gives AP and armor, Athenes gives AP and magic resist, Banner of Command gives AP and magic resist... and thats about it. Half of those are niche items. There aren't many defensive items that give resistances without sacrificing offensive stats.
A lot of times, especially with how snowbally the game has become, I feel the need to be grabbing more and more defensive items early in order to survive and wait it out until me and the rest of my team can do something.