Realistically, does any average consumer know what's on which circuit?
Spanning the split phase will screw you up, across breakers won't be fun but shouldn't pose any serious problems, as long as it's not in different sides of the split phase.
I'm pretty sure they say this because actually explaining what will work and what won't either requires significant prior knowledge of power systems, or a couple of paragraphs of explainers before you can get a rough picture of what the hell they're driving at.
Everyone I know who has used powerline, just plug it in and see if it works. Those who were lucky, say it's great and works without issue, etc. Those who were not lucky say the opposite.
I'm just over here watching the fireworks, eating popcorn.