That's really a "how long is a piece of string" question.
answer: twice as long as it is to the middle.
Tau don't have the volume of fire to take horde armies by just laying down shots, but they do have an excellent mix of mobility and firepower, which if used well with a properly laid out army list can do quite well against hordes. And that same list often works quite well with no or fairly minimal changes against non-horde lists, which is something that is hard to do with some other armies.
Also, every unit on their army list but kroot and vespids can get its hands on the pinning ability - that won't work against many orc/nid lists but the ones it does work on can be absolutely screwed by all that pinning.
I'll agree and disagree with some of this... because:
YES, Tau are VERY adept at dealing with horde armies. I do agree that their strength lies in mobile firepower - but it does have to be said that the firepower employed does generally tend to allow you to deal with numbers. basically, the tau have less mobility that the eldar, less out and out firepower (number of guns) than the imperial guard but have a middle ground that can make them devastating.
Whilst pinning is often a sore point against nids and orks it is not completely useless... those armies can find themselves pinned under engineered circumstances (which the tau can achieve through their mobile firepower picking off key elements prior to employing pinning weapons)...
however, when you look at the sheer volume of shots that tau armies can dish out its not hard to see that horde control shouldnt be that much of a concern to a well thought out army list... its target priority whilst playing that often causes the issue.
