While I owned a set of Monsoons and the kept my stuff dry for a number of years. Since replaced with Oltlieb Front Rollers (pretty much the same size). I don't know if I could have got away with them for rear panniers... pack very little and you might get away with it.
When I used them for rear panniers it was for around town errands and such.
I like the simple closure they offer... I like the fact that they kept my stuff dry. I didn't much like the mounting system... while it always stayed connected to the rack.... The mounting hooks are funky but functional.. the plastic that covers the hooks wears off quick. Makes for functional but ugly.
For just a little more I would seek out the Ortliebs... For me it was money well spent.
If money just isn't possible for them.... The Monsoons will serve you well for many years.
Hope that helps...