California in general positive.
California San Ysidro border crossing USA side, negative banned by federal agents, had been force to dismount and had carry the unicycle, in the case of a Q3 it is possible to avoid hand carrying it since it has 2 wheels it is possible to roll it without rider and pick it up 10 meters ahead and roll it again, I find that to be more dangerous than riding it. However rolling it without rider seems to be ok by federal agents and legal, you just can not ride it. Rolling it uphill it is a lot better than hand carrying it uphill.
Mexico San ysidro border positive, the ban is only on the USA border side of California, no ban in the Mexican side of the border.
Mexico positive, not sure about the law but it seems to be ok acording to a few cops I spoke, no problems, however be prepare for a 16 or 18 wheel, sidewalks and streets are in very bad shape, you must use sidewalks, no such thing as bike lanes, and in the streets it is the law of the jungle, better to use sidewalks.