I've flown it in the 737 a number of times so sometimes it does.

I see how that was easy to confuse. RJ's, of course, take a bigger hit when airport capacity is limited due to their relatively smaller seating capacity. MKE reliability has an even bigger issue, though, because it is well inside the beginning of the FYTTE arrival and is more difficult to fit flights into the flow because, at that point, the flights, from farther away, have already been established with the appropriate in-trail spacing. That affects MKE-ORD departures regardless of the size of the airplane.

The regional operator gets a portion of the gross ticket revenue rather than a flat fee for each flight.

Is there a way for passengers to know which flights these are?

We've had a lot of storms in the last month which have affect both airports simultaneously, so that might be part of it.

Not easily but they tend to be Markets where the express operator either sees more potential than mainline does not has other business reasons to operate them (such as a maintenance base). Timing may not be as tightly coordinated with mainline banks.

IIRC the last time I read a UAX operator contract (which was a while ago so things may have changed) for at risk flying the airline also has to pay for fuel which is covered/provided by the UA for "fee for departure" (non at-risk) flights

That makes sense. I was wondering if United ever uses a slot for UAX flight over a mainline, due to looking at irrop recovery stats. A 319 with mostly OD passengers being more easily accommodated then an E175 with 90% transferring to TPAC, once-a-day destinations, etc.

The software they showed off was pretty cool. I can only imagine how sophisticated their tools are now.