Shaymin is largely outclassed by celebi. Even better: you can run a sub baton pass set on celebi to make belly drumming much easier
Shaymin has niches over Celebi when used offensively, in that Seed Flare's power and potential drops help it wall break, it has room to run Earth Power compared to Celebi, Air Slash for opposing Grass type, and Mono Grass makes it less weak to Knock Off and Pursuit Trapping. If being considered for an offensive partner, there is legitimate reason to consider Shaymin over Celebi. Celebi's Psychic type, as
malomyotismon noted, also does give it some trouble synergyzing with most hazard removers, especially since Excadrill would make the core even more weak to bulky waters and grounds alongside Zard-X, who is DEFINITELY going to need hazard control to run a Belly Zard set.
One thing about Shaymin is that it can beat Scarftar, as opposed to Celebi having to Baton Pass out
252 SpA Life Orb Shaymin Seed Flare vs. 0 HP / 0 SpD Tyranitar in Sand: 283-338 (82.9 - 99.1%) -- guaranteed 2HKO
That said, SubPass Celebi might make a better partner to give Zard a buff after the Drum damage.