I tried to find a good thread for this, but there isn't anything recent discussing sixth freedom bookings, nor does this really have anything to do with COVID (as schedule changes have always and will always be issues).

Many months ago, a friend wanted to go to BCN with his mom for Christmas. They paid for J.

SFO-BCN on the outbound, BCN-SFO for the friend / BCN-CMH for the mom on the return.

AC was one stop, UA was also one stop. The friend (who is UA*G) had previous good experiences on AC, so he opted to go with AC.

A few schedule changes later, and their outbounds are:
SFO-YYZ-ZRH-BCN / SFO-YUL-FRA-BCN
Their returns are:
BCN-ZRH-YYZ-SFO / BCN-ZRH-YYZ-IAD-CMH

So the mom's one-stop became a three-stop. Yuck.

However, AC will not change the over-water segment to non-AC. I've escalated this, tried to get a JV airline, anything. But they won't do it.

BCN-EWR-CMH and BCN-EWR-SFO both exist. But they won't do it.

And because AC no longer flies to BCN, it's impossible to get this down to even a two-stop (for CMH) without overnighting elsewhere in Europe. Stopping in Europe on the same day means they arrive in YYZ too late for the CMH flight.

I don't even understand their logic here. They found a relatively loyal UA FF who pays for J, but wanted to fly AC (and, to be honest, still wants to fly AC, just not on this routing). And he is no longer willing to book AC to other continents, because the wrong schedule change will force an extra connection or two.

If he'd picked UA, I'm sure there would have been many schedule changes, but at the end of the day, they'd either be on SFO-EWR-BCN or SFO-FRA-BCN, and something similar for the return (likely EWR so they could stay together for the first segment).

Instead, they're likely going to go with:
SFO-YYZ-ZRH-BCN
BCN-FRA / overnight / FRA-YYZ-SFO/CMH (this one because they'd rather spend 20 hours in FRA than do a triple-connection)

AC can talk about the seamless transit experience all they want, but if they're unwilling to book on even JV airlines when they have a massive schedule change (and for an airport like CMH, it wouldn't take much of a change to break that connection), then what's the point? Just stick with UA.