Pseudonymity is very hard. It only takes one mistake to undo 20 years of perfect behavior. As Patrick pointed out in your other thread, it’s very easy to make mistakes when you are dealing with multiple identities on the same screen. Qubes makes it easier because you can use different window decorations. Ideally, you would use different machines and be required to physically move to switch identities.
Consider also that if any two or more of your highly-customized VMs were to be compromised by the same adversary, it would be trivial to link them together. (The same probably holds true for non-customized VMs due to hardware and os fingerprinting - so more physical machines are better and less cloned VMs).