Not if you upgrade to the '03-'05 exhaust cam.
In the strictest sense, they are less reliable than an XR, but it's like comparing bricks made of two different steel alloys and debating which one is tougher. They need attention, but not much, and even an anvil can be broken.
Great info above. To elaborate: The XR250 motor is the indestructable motor for the enduro class. While far as MX thumpers the YZF's have proven to be most reliable but need more care than the Xr and will require a rebuild faster than the XR...not that either need a rebuild quickly. The deciding difference between the two is the YZ motor turns high rpms, works harder with more aggressive cam profiles...therefore its just a faster running motor. Faster running motors require more maintenance.
With that being said the power differnce between the two motors is night and day! The XR250 motor is solid and delivers good performance for what it was intended to be used for...but the YZ is just way faster due to the CC's, the cams, the carb etc.
I have a 2000 426, adjusted the valves once and have remained in spec..change the oil every ride, filter every other, and air filter every ride and its never been anything but great to me. You get a 426 and you will not miss your XR one bit.