Correct me if I am wrong, but the lightning weakness shouldn't be a problem.Lightning GX attacks will do nothing to it, and Zapdos attack doesn't apply weakness or resistance. Celesteela's energy requirements allow it to be an addition to any deck that is worried about PikaRom.
It matters because Tapu-Koko-GX, Zeraora-GX and Pikarom can KO this for 2 energy with their non-GX attacks. Celesteela only protects you from GX-attacks, not attacks of GX pokemon. And you can just move it onto the bench with Guzma and its ability stops working anyway.