Getting 3 beeps on restart after RAM install... is the RAM really bad?
Your comment of "supposed to handle" is key here. Apple supports up to a specific amount of RAM in upgradable Macs. Now guessing Apple's motivations is generally discouraged here (and celebrated elsewhere...), it is not a stretch to extrapolate why.

The "supposed to handle" comes from at least one person reporting that more than the supported amount of RAM worked fine on their Mac. In some models perhaps a lot more than one. So putting more RAM in a system that will reportedly work with more than officially supported is not much more than "my cousin Jane put more than the supported amount of RAM in her 2009 iMac and it worked fine."

Apple likely did not set the maximum RAM supported arbitrarily. There are likely technical reasons why certain Macs only support up to a certain amount of RAM. And you are one of the 'lucky' few experiencing an edge case where manufacturing tolerances have conspired to make your iMac one of the unlucky few that don't support more than Apple's Officially Supported specification.

In this particular case there are two possibilities. One, that your particular iMac does not support the amount of RAM you wish to install, PERIOD. Or two, that the RAM Crucial has provided you is defective in some way (doubtful from your testing scenario) or just incompatible with your particular iMac. And perhaps another manufacturers RAM would work fine.

Personally I would go with number one. But if you have more fortitude than I you could try exchanging the RAM or getting a refund and trying another brand.