Hmmm, I have both old-school Airport graphite (WEP) and a fairly new Airport Express (WPA-PSK) with about a dozen chumbys of various flavors (CC, C1, C8, I8, I3, dash...) with no issues.  What failures are you seeing?

What we're seeing a lot in customer service is devices that used to work seeming to suddenly not work, with no firmware or software changes.  One common cause is the appearance of a new AP from a neighbor.

What sometimes happens is that you get multiple access points with the same (typically default) SSID, but different encryption methods or passphrases.

Even with the same SSID, encryption, and passphrase, there can be conflicts as the device connects to one, then later gets a stronger signal from another.  I have different SSIDs for each of the zones in my house - you typically want one device (whatever's acting as your router) assigning DHCP while the others passthrough.

The other common related problem is an overloaded channel - some Wifi channels are worse than others anyway, but when you get a whole bunch of APs sitting on the same channel, everyone suffers.  One problem is that some APs can be configured to randomly choose a channel - that's often why everything works, then fails when the AP restarts, since it's now running on another channel that might be of lower quality.  I noticed a serious degradation in my home Wifi when power was restored after an outage - while my device was on the same channel as before, so were a lot of other devices that weren't there before.

The most common successful solution to failing Wifi that Customer Service has is to "switch channels on the AP".  I've had to do a few times myself in various places I've lived as new AP show up.

Another common reason for sudden failure of Wifi in the device is simply a change in the geometry of the environment.  Wifi is *very* weak radio, and subject to all sorts of quirks - bouncing, echos, "static".  Older folks may recall the tribulations we used to have with "rabbit ear" antennas on TVs , and how arbitrary the position of the antennas would deliver a good signal - only to go away again if someone moved.

What also doesn't help is that microwave ovens typically generate a lot of noise in precisely the same (unregulated) frequency band as Wifi.  This is a problem with chumbys in the kitchen - depending upon the channel, it may lose a connection entirely when the microwave oven is on.