Or they would be engineered with huge advantages against an ecosystem which had never adapted defenses to them (typical of successful invasive species on Earth), and work on timescales much faster than typical evolutionary ones. These would basically be microscopic von Neumann probes, presumably with some kind of self-organizing emergent AI to direct their progress, not just spraying random bacteria everywhere and hoping for the best.
Only a tiny fraction of species end up as invasive, because the local ecosystem tends to be better adapted. As to ‘grey goo’ there is fairly good evidence that’s far less possible than you might think.
If you don’t stick with Hydrogen Carbon Nitrogen Oxygen you’re going to have a much harder time finding what you need to grow on earth. For extra solar stuff even those might be hard to find.
If you do stick with HCNO it's hard to get a leg up on nature without simply making a few minor improvements which has limited value outside specific situations. There are also a lot of tradeoffs with binding energy etc.
