Switching is one thing, but in a world where everybody runs ice and fire moves to counter dragons and Scizor(and now also Nattorei) those weaknesses aren't helping.Celebi has pretty good defensive typing - few will argue that Ground, Water, Electric, and Fighting resists are bad to have. It may be weak to 7 few types but it can always switch out of them.
Either Breloom is effective, IMO. I find SubPunch as more of a team player while Techniloom can hit hard right off the bat. It is depressingly slow for SD to be effective, though.
Not to mention with new guys like Sazandora, the new steel bugs, Zoroark coming in and the continued use of Tyranitar, who now tends to carry fire blast and ice beam as well, it becomes far more difficult for Celebi to abuse its good resists, because its of its glaring weaknesses.
The new generation really isn't helping its cause as a defensive pokemon, much more so than gen4.