Despite best efforts from industry standard groups, interoperability among Wi-Fi products is hit or miss these days: Not all products are certified by the Wi-Fi Alliance and some vendors are cutting corners. For example, they may not do a great job of implementing the mechanism that asks an 802.11g chip to fall back to 802.11b.
New security features and the plethora of chips available increase the complications in making all devices compatible.
I don't imagine the compatibility issue will get any better as we move into the future because more upgrades to the standard continue to be developed.