Earning money with Face of the Mountain becomes extremely difficult during late game
During the laning phase, Relic Shield and Targon's Brace are a joy to use. Smack a minion, earn some money, heal yourself and your laning buddy. The floating charges around you are a fun little resource to manage--should I spend my charges now, while we have our opponents on the defensive, or hold out for a siege minion later?
But during late game, spending those charges becomes hugely frustrating. No longer can you easily finish the minions your carry has started. Now your carry is a minion-killing monster, chewing through creeps like a harvesting machine going through a cornfield. Executing minions that your ADC has wounded becomes nearly impossible, because they can drop a minion from 100 to 0 before your autoattack even lands. And the damnedest thing is you made them this way. The better job you do as a support, the more ferociously they'll be able to CS, and the harder it will be for you to get any gold out of Face of the Mountain. So the four Orbs of Shame will float around you until the Nexus explodes, taunting you with visions of gold that you can't have.
Lots of people will probably say "Just sell the item when you can't CS anymore." But for all its problems, I like Face of the Mountain. I like its stats, I like its flavor, and late game, when you have loads of health, is when its active is at its most powerful. I just don't like begging my ADC to leave the cannon minion for me so I can beat on it for 10 seconds while he stands there awkwardly waiting for my item to do its job.
Surely it wouldn't upset game balance to raise the execute threshold on Face of the Mountain so that supports can spend their orbs late game?