the cost is really the main problem...
if you think about it, in the unupped field of play, what is the most common creature? shrieker, phoenix, lava golem, etc. all of which are very fragile, and that is why unupped plague costing 4
is bad, it takes more than 1 turn to kill anything, where as RoF, or panda take just one turn (panda costs less too, and with immo, RoF isn't as expensive as it may look)
which is why changing the cost to 3
will make it more effective
the upped version is fine, it's cheap, and in upped field of play, firestorm and panda are less effective since creatures gain a little more hp, but plague will kill them eventually