I can live with that, it would just be nice if I could fly with Odyssey commanders where that gameplay overlaps. After all, base game Elite and Horizons CMDRs could fly together in space, even though people tell me those were also two different game engines (IDK myself, having only played Horizons).
Thankfully I'm still finding some players in Horizons at the community goals, though it's obvious by the system chat that many are now in Odyssey. It sounds like all the gankers moved to Odyssey, so at least there's a silver lining to every cloud
There are issues you couldn't have with horizon vs regular though. For example, planets are different in odyssey. Say we fly together to a planet, there is a mountain for you, but not for me, so I go through it on your screen.
And then I can drop outside the ship on leg. And you can see me because you're landed near me. But the horizon client probably doesn't have the Odyssey files for all the spacelegs textures, mesh and so on. So you'd probably crash.
Not to mention new stuff like the scorpion SRV which wasn't added to horizon.
Eventually, it would only get worse, because when they add stuff, it will be for the odyssey branch of it, and since horizon will never have the file for it, that would create issues.
Which is why, I imagine, they still want to merge odyssey and horizon, so the playerbase on PC is not separated anymore. And so in the examples I said earlier, you wouldn't have issues, since the mountain is there for you to. And even if you can't go on legs, you'll have the files for the character, guns and whatnot. So it shows on your screen.