Writing for Intel does increase one's problems exponentially, because now you also have to troubleshoot someone's hardware configuration. And you're right, to be efficient, Apple writes its OS with a deep knowledge of the hardware. This may not necessarily translate to the same performance against two different Intel boxes.
I can't say if Apple is as bad as Be when it comes to knowledge of the hardware, but I was extremely disappointed in BeOS. It was so cool and had so much promise, but I couldn't get it to install on one of my systems. I bought at a CompUSA 2 Toshiba Infinias on the same day. I installed BeOS on one, it worked like a champ. It would never install on the other one. I spent hours and hours and hours trying to figure it out- after consultation with Be, it turned out that there was a tiny rev difference in the chipset between the 2 machines. That was enough to make BeOS install on one Toshiba and not on the other. No wonder Be never made it! If Apple is that dependent on the hardware, it will never make it in the Intel arena.
Still, this is not rocket science. There should be a way to write an OS that can work in either Intel or Motorola environments - that is, assuming Apple wanted to do that.
On the other hand it ran like a bat out of hell (for the era) on a stock HP Pavilion 83**. R5 which was the last version I used ran circles around my friends' custom built boxes that had athlon 600-700s running WinNT/2k on my old 8390 which had a P2 450 and 128mb of ram.