The land of cactus is where I want to be. How fortunate indeed!
The rules on border crossings probably vary wherever you go. The border crossing between San Diego and Tijuana is the busiest one in North America. Some insane number of vehicles pass through there every day of the year. Going north or south, the authorities are concerned about soil and they are concerned about plants (separately but equally). Presumably there are potential disease issues with both, on top of the endangered species rules or whatever.
I've heard the US CBP will bust you for importing soil if your offroad vehicle is plastered with a sufficient amount of mud, that may be an urban legend or actually true. But regardless, a great reason to wash your car.
The enforcement does vary. American law enforcement is very careful to inspect everything, the Mexican customs guards more or less wave you through (or ignore you completely while they work on somebody else's load of contraband). One time I saw them working on a giant box of tiny baby turtles, like hundreds or thousands of them, obviously that was kind of an event. They probably have seen everything.
In the last couple of decades (more or less since 2001) they have gotten a lot more serious about checking people and vehicles. Obviously drugs and people go one way, guns and money go the other way.
Decades ago there was actually a fair amount of cactus/succulent extraction from habitat, going from Baja California through the border crossing into San Diego and points north. I've seen ancient plants displayed to me proudly that were dug up as adults and transplanted in San Diego County, to be in someone's garden. I think the border crossing that matters most with respect to plant conservation is the one into the US, and I'm happy to see these things taken seriously there, however much that might conceivably cramp my style.
