Assuming they are reliable. the usual 1.1V reference an AVR is +/-0.1V!
The ATMEGA series has notoriously bad analog parts. The "10 bit" ADC is barely worth 8 bits, and that's if you believe the datasheet.
I've also heard that the POR function fails at slow supply startup (dV/dt). How do you screw that up?
The XMEGA series is much improved, having more, and more powerful, peripherals. The ADC is 12 bits, and actually worth most of them. It's 10 times faster. The analog comparator is >100 times faster. There are more references, that are actually fairly stable (though still nowhere near even the humble TL431).
Alas, XMEGA is more expensive than ATMEGA, which is overpriced already. If they could cut their production costs they could bring them to compete with PICs. I suppose it will be interesting to see how Microchip decides to play them out, if they'll leave them as they are and "let the market decide", or shift market share to optimize sales figures, or discontinue one or more outright (which is really just saying the same thing).
I don't get the feeling that any similar PIC can compete in terms of power and ease of use -- given I'm not deeply familiar with the PIC family. I know they go up in bits, but now you're talking a different segment, and they still suck: few registers, quirky instruction sets, huge errata sheets. And then there's the PIC24 abominations, and then PIC32 which is MIPS and therefore should compete with ARM and such instead (though I don't know if they have any that are nearly as powerful as Cortex >M0s). So, it seems like AVR is a value to them, to fill in that hole between crappy 8-bitters and nice 32-bitters.
Tim