Really tough to pick a scenario where you wouldn’t want the Toad, to be honest. I had to think pretty hard.
Basically the only hypothetical scenario I could come up with is if you have the first enemy as a priority target (i.e. fully charged with something fairly dangerous) and one of the other enemies is fully charged with a skull generator - but you have to hit the first one. In that case you would prefer the first troop to be entangled when the skulls go off, instead of being a Toad that isn’t entangled. However, you do explode the board with EOE, so whatever problematic skull line-up you’re looking at becomes a crapshoot (and you might even get an extra turn). Failing that, hit the skull generator with EOE instead.
Otherwise? I really can’t think of anything else. And it’s pretty low odds on the first one already.
I mean, yes, I’ve had my share of “oops” moments with that, but nothing critical and definitely not outweighing the general usefulness of transforming them into Toads.