not if you use waterwalking with the cultists and approach from the water, then they have to have a navy to defend against it. Oh they have a navy? Just summon the big squid creature to protect your Cultists while they spam on the Tsunami, easy as pie! And yes, Tsunami can hurt your own units, but if you play it smart, you can position the Cultist so he can hit a city without hitting the tile your units are on too.
Tsumani has the same range as Maelstrom, so theoretically the enemy can bring over some air mages and strike back with maelstrom against you, weaking you enough to then attack, with say, fireballs or a fleet.
I agree that Tsunami is very powerful, but a good player (or good AI) would know how to deal with the situation.
One thing I dislike about Tsunami is that if there are any units from other civs in range, using the spell is a diplomatic nightmare. There have been plenty of times I wanted to use Tsumani but didn't because it would cause a war with a third party I was not interested in being at war with at the time, including vassals or mine.