yeah the bog beasts are quite cheap
only thing which doesn't suit me very much is they are strat move 1 only . so if the front is far away you either use them for defensive purposes or get the summoners first to the front
but as you said gemwise it is one of the better spells you get quite much for your investment
there are no nations with 100% natural poison resistance units or ?
expect pythium serpent cult with masshydras .