Titanium is a brittle material compared to aluminum, and those macs had known issues relating to the hinges for the screen breaking. The other issue is that since paint does not easily bond to a titanium surface, many of those Macs had major issues with the paint rubbing off in spots all over the machines, especially on the corners of the devices.
Titanium isn't necessarily brittle - that depends on the alloy and especially how pure the metal is. High purity titanium (and alloys) can be very tough.
However, that didn't matter because what broke on TiBooks wasn't the titanium, it was plastic. The titanium parts were two stamped sheets: the bottom cover and the palm rest/speaker grille. These were not the primary structure of the computer, they were really more like skin on the frame. The frame was a big ring of some kind of fiber reinforced plastic. If you look at a picture of the TiBook, it's the whiter substance providing some visual contrast from the silvery paint used on the titanium. (Both colors were paint, not the actual material color, so paint chipping affected the plastic too.)
The mechanical stresses associated with the screen hinges went through that plastic frame, and it would frequently break at the hinge attach points. Another weak point was the front of the machine, where they molded a giant slot for the DVD drive into the plastic. And another was the hinges themselves, which were metal behind painted plastic housings, but prone to breaking. Basically, everything broke all the time except for the actual titanium parts.
