He is correct that Seraphim is quite weak in the T2 stage, but I don't think it is any worse than Cybran (which everyone thinks is OP). It is more an issue of UEF being too strong on the T2 stage since they get mobile shields + mongoose, pillars, and a very strong ACU all at the same stage of the game. Similar issue with Aeon.
I think mobile shields need to be nerfed in some way. They could be made to die when the shield bubble is overcharged, which there is already a PR for on GitHub. Another potential addition to this would be to give them a death weapon that activates when they are hit with overcharge, or other weapon that deals large overkill damage, so that they damage nearby units when they die. As if the shield was being overloaded by this additional damage or whatever.
I admit I was incorrect about it taking more than one energy storage to kill those units and I should be banned for spreading this misinformation. I was confused since it does take more than the minimum required overcharge energy to kill them and I mistakenly believed keyser knew what he was talking about.