I've only had one issue with multiple screens and nvidia - it doesn't always save the order of the screens. Otherwise, the driver works flawless for me. KMS has been nothing but a pain in my rear (its introduction, along with the dropping of DRI1, forced me to stop using Linux on quite a few laptops that I had/was supporting at the time) and the fact that 'nomodeset' is required for nvidia is a plus in my book. I really can't speak about proper XrandR support as I have never dealt with it directly.
Different archs? Yes, support for Linux on some platforms is lacking (no nvidia on PPC, I've not seen an nvidia card in a MIPS, POWER, or IA-64 system so not surprised there is no driver support there), but nvidia supports more then fgrlrx does (nvidia works on Linux on ARM, plus FreeBSD AND Solaris on Intel).