We'd assume most interstellar objects are comets because that's which objects you find on the outskirts of a solar system and are the easiest to get kicked out. We'd assume they're mostly like our comets due to the Copernican principle. We shouldn't assume we're special.
> We'd assume they're mostly like our comets due to the Copernican principle.
We're still figuring out what our comets are like, let alone unusual ones spending a few million years in interstellar space. New types of comets(ish) bodies discovered in the 2000s:
https://en.wikipedia.org/wiki/Active_asteroid
https://en.wikipedia.org/wiki/Manx_comet
We've spotted ~5k out of an estimated trillion. Each one we've sent a probe to has brought surprises. The Oort cloud remains theoretical at this time, and the first Kuiper belt object other than Pluto/Charon was found in 1992. It would be deeply silly to think we know everything about our local comets, let alone unusual ones from elsewhere.
Why would we assume non-interstellar comets are always the same as interstellar comets? Conditions obviously are a little different when something is ejected from a system and then spends millions of years in interstellar space.
> Borisov had the same characteristics.
We have a sample size of three thus far. Making conclusions right now is like saying all extrasolar planets are large gas giants because the first three were.