That's not my experience, over the last 15 years where I was required to exchange PKI encrypted emails with both DoD users and other contractors (Fortune 50 company through 1 person security consulting shop). I've had problems setting up/loading certificates, particularly handling root and intermediate certificates (from DoD PKI). When a certificate expires, Mail has real problems with the email. And recently I was sent a short encrypted message where it took order a couple of minutes to decrypt and display.
Those problems, I believe are a combination of flaws in Mail.app, in the underlying Mac OS X PKI support, and with PKI in general. I had similar problems with Thunderbird, which depended on little or no Mac PKI infrastructure.
Hence my posting elsewhere in this thread that it's the underlying PKI infrastructure at the OS level that is at least partly at fault, and I think the complexity of the PKI design explains much of the reason why PKI infrastructure is so messy. What looked good on paper didn't scale and had real usability problems even for relatively sophisticated users. It's certainly not ready for the casual user!