The famous weakness of blue is that enemy creatures cannot be destroyed completely
Unfortunately, he doesn't have many answers to control or removal effects, and the low-cost nature of the cards in his deck makes him vulnerable to some specific strategies
$0.35 Typhoid Rats Fate Reforged $0.35 Persistent Marshstalker Bloomburrow $5.99 Lord Skitter, Sewer King Wilds of Eldraine $0.39 Revenge of the Rats Foundations $1.49 Gnat Miser Saviors of Kamigawa $0.59 Voracious Vermin Wilds of Eldraine $0.79 Piper of the Swarm (Bundle Foil) Promotional $1.29 Tangled Colony Wilds of Eldraine $0.39 Pestilence Rats Ice Age $0.99 Rat King, Pale Piper Teenage Mutant Ninja Turtles Eternal-Legal $0.49 Rat King, Verminister Teenage Mutant Ninja Turtles $6.99 Rat Colony Dominaria 0 results found
Basically, we're okay with a few cards that put +1/+1 counters on creatures but we tend to avoid more complicated versions (such as 0/0 creatures that come into play with +1/+1 counters) at common, although we're more willing to do something like that if there are a number of cards dedicated to a certain theme ( Darksteel 's modular, I'm looking at you)
Overall I would say that simply being blue adds more than the multiple targets potential does for the card as per the previously mentioned Force of Will, Chrome Mox etc uses
