Well Phione even finds a hard time in the NU tier so it can by far not do as well as it's Mother/Father figure, Manaphy. Unlike Manaphy it has a shallow movepool and a rather numb stat collection as any supereffective hit will most likely bring it down. Due to the fact that it needs to set up so even become a little useful in the NU tier, many Pokemon can easily taunt or set up on it....even Butterfree though Butterfree is still a notable threat in the NU tier.
The NU tier is also full of Pokemon that can profit from status inducing moves such as Toxic tha will enable them to kill it after an attack/speed boost.
This is just for the NU tier
In the higher tiers it is outclassed by many other Hydration users such as Vaporeon and Lapras and isn't bulky enough to take a hit from the Grass/Electric type Pokemon or even neutral attacks and can therefore not stand as a Pokemon in a higher tier. Though it is still possible to balance it's moveset in the NU tier, this is done with a lot of caution and it's only effective stat is completely outclassed and can be stopped in even the NU tier. This shows that it isn't ready to take on the higher tiers