I would have said that too years ago. I always had older cars with mechanical linkage clutches. But I have been driving several later model trucks with the hydraulic setup, and it's pretty reliable. I really have never had a failure after years of use on my diesel and my little ranger pickup.
I've had two failures of IDI/zf5 hydraulics:
On my 92, after sitting for 3 years and having been parked because of clutch issues, it had no pedal(when I bought it). Filled the brake fluid, and it bled and worked, though lost prime really quickly. Replacing the 25 year old slave cylinder cost maby $50 and took an hour - No issues after that.
On a friend's 94, I found that someone had added a "shield" of phenolic between the plastic clutch line and the exhaust manifold, which eventually wore through the line and caused major leakage.
A new line, not particularly expensive, fixed that right up.
This is out of a sample of 8 manual IDIs I have or work with... Age is definitely a factor here.
I've never had one fail on me, and I like how they work.
edit:
Looks like I responded to an old post in this thread. Still, point stands.