[cnvogel]

What other people have already mentioned: They are much less performing than x86/amd64 currently, and a lot of the ARM cores really suck these days when you want them perform CPU-intensive stuff: My i.mx6 and nvidia-tegra overheat without additional cooling. And look at the raspberry-π: The ethernet-controller/usb-hub combo eats about as much power as the CPU!

Then, the state of "board support packages": In contrast to x86 you cannot run a linux-kernel as-is, but rather it has to be patched for that particular board (in most cases): The linux-kernel has to be supplied with a lot of information on compile-time that is commonly provided by the BIOS on your typical PC. Furthermore a lot of the peripherals present inside those tiny ARM-chips have to have their drivers added:

Now the Raspberry-π again, due to its popularity and enthusiastic community excels: You can basically get distributions with modern (now: 3.8.x) kernels ready to install. But if you are stuck on i.MX6, you get to use a patched 3.0.x-kernel (+2000 patches), with nvidia tegra it's at 3.1.x. [at least that was my experience with industrial boards up to a few weeks ago].

  - https://community.freescale.com/thread/300881
  - https://developer.nvidia.com/linux-tegra

And those kernels tend to break in interesting ways as soon as you try to add your specific features, like getting a certain PCIe card to run with the i.MX6, or fiddle with the i2c-busses (on which power-management chips are present), ... and just are much, much, much less well-tested as the mainline kernels you are used to on "normal" PCs, or even the other more mature mainline architectures such as PowerPC (does anyone still use sparc/m68k/...?).

{Side-note: That's also the main pain for the independent android ports, such as cyanogen-mod. They normally get stuck on missing kernel patches, sourcecode, ... even though it's just "an ARM" chip.}

Of course, this is not at all a problem if you are happy with the stock BSP kernel/distribution and just want to compile node.js or mplayer for your board.