I'd say that's debatable--at least the part about "whatever timing" and it being "virtually guaranteed to work." I mean, if you use the right software and interface circuitry then a kitchen table EPROM programmer is going to work, too. And if you don't get the timing or software right, an Atmel SPI-based ISP isn't going to work any better than a broken kitchen-table EPROM programmer--as witnessed by so many people having problems with Atmel ISP.

Like I said earlier, I just see the difference between an EPROM programmer and an ISP "interface/programmer" to be a matter of degree. ISP interfacing is going to require fewer components than an EPROM programmer but you still can't build the circuit incorrectly, use the wrong software, or use the wrong timing and expect ISP to magically work whereas an EPROM programmer would fail. They'll both fail under those circumstances.

Regards,
Craig Steiner