I have read about one case that was denied because of it being unclear how the people would support themselves. In that case, the husband & sponsor was making very little money for the last few years, working in a restaurant. I think about $8000 a year so obviously he can barely support himself from that. The wife spoke very little English so immigration thought it unlikely that she would be able to work.
I think it was you, Leon, who first alerted me to this case.
But it DOES seem to me to be a
de facto minimum-income requirement. If the sponsor cannot support himself and spouse, then CIC won't award a spousal visa. It's just that the "minimum" is not stated objectively, and so may be up to the subjective judgment of the VO. And CIC seems willing to accept the would-be sponsor's assertions about income. In my case I did not provide objective proof; I provided only a spreadsheet showing income sources.