It's kind of an ongoing debate as to the risk/reward ratio for using an U21.

People who say always just get an O21 will point out:
-Some bars and stores may just reject all vertical IDs due to improper training ("All vertical IDs=minor" is wrong but told to some employees)

-You may be asked to produce secondaries or more backup forms due to it being U21

-Have to find a state that gives you a long enough validity term so you are limited to a set of states you may not have been interested in

People who argue for using U21s will point out:
-Bars are less familiar with them causing them to not recognize as many subtle differences as they would w/the O21 version
-Slightly more plausible for younger looking people

-It has a sort of "Big dick energy" to it that helps with confidence, the assumption is that if they are going to show an U21, they are already ready for the ID to be examined for double checking DOB and EXP (The two things bouncers have to be careful w/ on U21s not matter fake or real) so the idea is that why would they bring a fake that is going to be looked at more?

Those are both sides of the argument as I see it, rn there are some U21's on the market but for each one the vendor also has an equivalent O21, so it's really up to what your uses are and how you feel about the above points.

HomeDepotEnthusiast hit all the major points if you use a dob that is over 21 on a u21 id. I think a lot of offerings for u21's are for Tobacco/vape/lotto stuff and people in Canada that only have to worry about an 18/19yo age limit.