That's probably the case for the majority of planets, also even with multicellular life there is no guarantee that intelligent life would evolve. Earth went 4.5 billion years with 700 million with multicellular life and imagine if early hominids got wiped out, would another intelligent species follow our trajectory?

Also I would like to point out that the vast majority of planets we detect happen to be around red dwarf planets since we need 3 transits for a positive detection which would require years for larger stars. Additionally to do atmospheric spectroscopy the solar system needs to be within a perpendicular line-of-sight of our solar system.

So keep in mind the planets/solar systems we are studying do not represent the typical solar systems around exo K/G type stars.

Realistically, it's very unlikely we'd have compatible chemistry for them. It's like, our bacteria would be hopeless against ammonia blood or something.

But fungi/protozoa are probably a better analogy... they're more generalists and would stand a better chance of being invaders. Against something totally alien, if it could survive or thrive in an animal, I'm skeptical our immune system would be worth much, but I don't know.