Yes, you're right. But one benefit would be that if you earn *G via UA, then you will have access to AC lounges whenever you're traveling domestically in Canada. (I don't know whether the same would be the case if you earned *G on AC).

But I would suggest that you consider actually using UA for some of your trans-continental flying. It would require an extra stop (at ORD or IAD) en route to Europe or the Middle East compared to AC (although LH doesn't serve YUL-FRA, either). But the benefit would be that if you take the IAD-DXB or -KWI flights, having UA SWUs would give you the added benefit of upgrades to F, which, while not exactly world-class, is probably better than AC/LH C on the North America-FRA/MUC-Middle East routes.

I say all this out of ignorance of AC's program, so it might not make too much sense. And, of course, you could fly AC straight from YUL if you were an Aeroplan member and flying paid C anyway and perhaps be better off.