Arduinos powered by AVRs aren't high-frequency. The highest-frequency external component might be a 20mhz crystal, which you can lay out just by following the example design in the data sheet.
Most ARMs up to even a few hundred MHz are also pretty easy to lay out.
For wireless you're right, but this is more or less solved by the recent proliferation of cheap plug-in wireless modules that are already certified.
Using a module or not, you still have to go through unintentional radiator testing, though, right (if you have a clock signal on the board over some ludicrously low frequency)?
Most ARMs up to even a few hundred MHz are also pretty easy to lay out.
For wireless you're right, but this is more or less solved by the recent proliferation of cheap plug-in wireless modules that are already certified.