Is there a technical reason not to upgrade a MacBook's RAM past its listed maximum? According to this page, Macbooks are expandable to memories larger (and, in one case, faster) than those released by Apple. Search for "expandable" in that page to see the many cases.

But Apple is keen on bleeding edge technology and specs, which suggests that Apple has a technical reason (odd crashes?) for not offering the largest/fastest memory, not even as an option.

Would you comment on the reliability of notebooks after performing these upgrades (i.e. have you experienced increased crashes / instability)?
Most new Macs have memory that’s not packaged for mechanical installation. The exceptions to this are the MacPro and 27” iMac and Mac Mini with Intel CPU.
If your machine has end user replaceable RAM, anyone that is skilled at troubleshooting bad RAM shouldn't shy away from learning what they give up when they install third party RAM in their Mac.
Basically, you give up having AppleCare or the Warranty cover that part and perhaps them diagnosing a problem they can't reproduce with factory RAM installed.
As you probably know, an intel designed (or influenced) chipset and logic board will often run with more memory than Apple will install in the factory, as an upgrade or even list as &quot;supported&quot; yet reputable memory vendors have been providing details on which machines will run reliably when loaded with high quality RAM that meets the technical requirements if not the marketing requirements from Apple.
I would look to sites like RamJet or Crucial or OWC/MacSales that have covered aftermarket upgrades to Macs for years and have established a reputation for having good warranties and information on determining if a specific RAM configuration is expected to be reliable on a Mac.
You of course may run into issues and bugs where certain chips fail oddly or tuning isn't ideal, but the time you gain in having more RAM often outweighs the time / expense of doing some hardware support if you run into issues and want to swap back in the Apple factory RAM to ensure you're not to blame for any oddities or failures by pushing the line on RAM.