Right now, early adopter states can force the driver to go to the hospital to have blood drawn; and, if any presence shows up, they can be charged with DUI (CO) because there is no official standard for intoxication - you are DUI if there's any evidence of 1) marijuana possession (including officer's nose) and 2) you ever corrected suddenly or reached the lane divider marker. So the issue is not that DUI is a crime, but that it is defined so broadly for marijuana that any vehicle operator is guilty of it by essentially subjective testimony.

However, imagine now we go national with the status-quo of CO (no roadside test, zero blood sample tolerance for DUI once PC established), other states following the early-adopter laws since, as you said, things are going smoothly enough in those genius-run States. Now, the financial incentive of added taxes and tourism revenues for soft-touch enforcement by the early-adopter States changes and the problems are replicated nation-wide.

Would you really like cops everywhere across the nation to be able to stop any driver they want, search the driver for officer safety during transport, hold the driver for the period of time to arrange transport to the hospital and medical services, have the blood drawn (up to 2 hours detention legally to this point), and then await the blood test results, then finally arrange the completely-clean, non-using driver (actual users now presumably have a court date and bond-arranging to look forward to) to be returned to his vehicle which was either left on the side of the road or towed someplace based on issues of traffic safety - all based on nothing more verifiable than "the officer swears that s/he saw the car swerve and smelled pot"?

Can you see where this simple 'investigation' just by its nature could be abused as a form of intimidation, revenue enhancement, or even by a lazy officer who wants to be off-calls for several hours while he baby-sits the driver at the busy hospital?

How does an occasional user ever dispute the officer's claims of intoxication PC since detectable traces of marijuana use can remain in the system for weeks?

Can you see any civil liberty issues with blood being used as a DNA evidence comparison once they have drawn it legally as part of their investigation, even if you are released later as sober?

Does having this DNA evidence become a bigger problem if it is more widespread? Would you trust State and Federal government... yes, I'll go there, even Kamala Harris... with a criminal/medical database of every vehicle operator's DNA if they could legally obtain it over time?

How hard would it be for corrupt officers to divert or later synthesize enough trace DNA to frame somebody?

And that's just some of the government side's unintended consequences, without any business issues e.g. your company firing you as a driver/operator because you now have a DUI liability for them.

Does all of this above maximize liberty?

It seems to me that the lesser risk to liberty is continuing the status-quo general Federal decriminalization while court cases in the early-adopter States and on-going research mean that we can have better intoxication definitions established and roadside testing developed so that the worst case for abuse is the equivalent of an alcohol DUI checkpoint and not a 4+ hour odyssey in police custody for the squeaky-clean non-user with a bonus charge(s) for any user with any detectable trace of marijuana in his/her system.