Manaphy EX is going to be a mandatory 2-3 of in water/every deck in the future, calling it now. Think about it the last 2 promo cute/small legendaries we've had were mandatory 1-3 ofs in every deck aka Jirachi EX and Shaymin EX.
Not enough data points for a sound conclusion; after all if we go back a bit before we get
Celebi-EX, which has only seem very minimal, successful play (if at all) in specialized decks. Of course if you just want to make a somewhat bold prediction... I fear it is not overly risky.
When a card is so hyped like that, you need to proxy and check it out before getting too dramatic.
I agree with not getting overly hyped
however once you do learn how the game functions well enough you should be able to spot potentially problematic cards... and the reason they are potentially problematic is a small part "Huh that doesn't work out as well as it did on paper." and a large part "Oh, but they also released [something to balance it out... or that is simply even more powerful] alongside it." In this case we certainly do not need anymore "easy acceleration" and if it fails, it is still not a balanced card; we just have even more powerful forces at work, crowding it out of decks. ^^' No really, we've seen that for
years;
Crushing Hammer was preceded - what, a decade ago? - by
Energy Removal 2. Functionally the same card except for the name, but I don't recall anyone really running it. Now? It is actually less played than it was a few years ago when Hammer Time was huge!
So yes, testing is good, but you didn't proxy the entire set and remember real testing is
tedious with how much effort is involved; dozens (plural) results are needing even for small things.