Also the lizardman got owned when the skaven blew up the moon
As far as I know launching a moon at a planet is not a daily occurrence, even in 40K. Don't get me wrong, say a full on orbital bombardment with an entire fleet will be just as dangerous as a moon falling from the sky, but a few missile batteries will not even begin to compare.
Also, I am assuming that the Tau do actually want to keep the planet they're fighting on. Obviously if they start virus bombing it or other forms of exterminatus the Seraphon won't last terribly long.
Not to mention that magic is prevalent in the form of psykers already, so a wizard doesn’t bring anything new to the table for the tau even if the slann are the best at it.
For starters the tau have no psykers themselves, which is of course an advantages for the seraphon. However, a minor wizard like a Skink is indeed nothing too special, the tau have dealt with psykers that merely throw fireballs around before.
However, the Slann are far more powerfull than an average psyker; when that moon fell from the sky Mazdamundi was disintegrating continental sized chunks of it. Kroak is just flat out refusing to die cuz he's too busy murdering daemons for eons on end. That puts them on the level of alpha & alpha+ psykers, those can conquer planets on their own and take down titans.
Slann would basicly all be alpha or alpha-plus psykers maybe beta at worst. And unlike the regular alpha's and alpha+'s in 40K slanns are actually in control of their powers and sane. All slann working together should definitly be a considerable threat to a planet or system in 40K.
not mention the near endless numbers of them from all around the galaxy.
Again, I am assuming we're limiting ourselves to a "fair" battlefield, so about 1 planet's worth of Tau against roughly 1 planet's worth of seraphon, or maybe a system or two worth of Tau & seraphon. At which point it's similar numbers as trying to hold of a minor waagh invasion but with far better magic & far better organisation. Which should be a considerable threat to 1 planet's or a system's worth of defenders. And if the seraphon would happen to invade a non-military world they'd win easily; a local PDF doesn't have the firepower to deal with this level of psykers.
Obviously on a galactic scale the Seraphon would lose, even against a minor galactic empire like the Tau, unless they are allowed the time to slowly build up their numbers and conquer the occasional planet until they themselves become a proper galactic force with a viable space fleet and either the technology or numbers to not need to rely on Slann for every major engagement.
Also, if you find orks a bad example cuz they break the laws of physics, then compare it to a rather minor tendril of a hivefleet landing on a planet. Yeah its probably not enough to conquer a fortress world, but it's definitly going to cause significant damage. And a less defended world is definitly going to need help.