Invasive species are certainly a concern with a lot of things: just look at how Kudzu has taken over so much stuff in the American southeast. However, cacti I

Invasive species are certainly a concern with a lot of things: just look at how Kudzu has taken over so much stuff in the American southeast. However, cacti I would think would be unproblematic, especially saguaros. Saguaro cacti are huge when mature, but it takes over a century to reach that size. They're very slow-growing. They're not going to overtake any land on their own unless you intentionally ignore it for a few centuries. Moreover, they only do well in the Sonoran desert where the climate is right for them, and there's no competition or predators. They wouldn't last anywhere where anything could eat them (not likely with the spines of course), or where the weather killed them off (they probably can't handle freezing temperatures very much). They can't even handle too much wind: their root systems are very shallow, and they tend to fall down in storms when they're old. Young and middle-aged ones that are transplanted are normally propped up with 2x4 lumber for a while to let their roots grow more; it's an extremely common sight in Arizona in places with new development where they transplanted some cacti in for landscaping.