Let's talk about Shen's ultimate.
Stand United. Your allies are happily farming in bot lane, raging at each other peacefully with passive-aggressive pings about the support wanting to get relic shield procs on cannon minions when all of a sudden, the enemy jungler ganks bot. Now, you're wondering... how would they handle this since bot lane had already flashed in greed to try and get first blood? Well, fear not, my friend, since your top laner decided to play Shen instead of Fiora top. So you see your screen turn purple in the middle of your fight, Shen is coming to save the day.
You die.
What happened? You were level 6 while the enemy was level 5, the jungler was a level 4 Master Yi. Why didn't Shen's shield save you? Well, here's the real question.
Why can an ADC/melee carry tear through a global ultimate shield in 2-3 auto attacks?
250/550/850 HP shield. While it's all nice and dandy that Shen has a global teleport, 250 HP doesn't cut it anymore. Carries deal way too much damage in this meta, and aggressive junglers even more so. I've seen a level 6 Zed shred through the shield with one E-auto and kill my ally, who still had a third of their HP. I've seen the lucky crit of an ADC make the whole shield disappear. Late game, 850 HP is 2 seconds of an attack speed carry, or an Ahri Q. The ability is powerful, don't get me wrong. But the shield doesn't do enough to keep your allies alive.
#The Solution Here's the problem: the shield isn't enough to keep carries alive. Should we buff it? Hell yeah. Give it a good 50 HP at rank 1, ramp it up to 1k late game so that you can keep your carry alive for at least a second against that Master Yi. So... what should we nerf in particular? I'm going to piss off a lot of people when I say this... but AP Shen is dead. The only scalings he has are a 1% per 100 AP max hp damage on his Q and a 1.3 AP ratio on his ultimate. So, let's get rid of the AP ratio since Riot clearly doesn't like AP Shen, and just go the full mile to making him a bruiser/tank. Buff the shield values, remove the unnecessary scalings. Nobody will play AP Shen even during URF because going full tank gives him more damage on his E and more shield with his passive.
Now here's a random poll for your benefit.

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the f@!% off.