Thanks for the help.
When I use your example, it still doesn't work as I'd expect.
[[!If?
&subject=`[[*latestNews]]`
&operator=`isempty`
&then=`[[$latestNewsMIGX]]`
]]
The above doesn't display anything, whether there's data in MIGX or not.
[[!If?
&subject=`[[*latestNews]]`
&operator=`isnotempty`
&then=`[[$latestNewsMIGX]]`
]]
And this one displays something no matter what (either just my sidebar title/background, or the title/background plus MIGX content when there is some).
I've tried the calls cached/un-chached, and with clearing the Modx cache and my browser cache.
Is it possible that MIGX is passing something along not matter what? Even if it isn't actual displayed content.