Specifically, Viper has two glaring flaws, both of which are not suffered by E, Monte, etc. The first is a lack of correct encapsulation. Viper appears to have a Python-style object model, which means that it's not possible to closely hold a value. As a test, how would you build an object with a value in its closure which no other object can access?
Second, Viper doesn't have mitigation for plan interference, which means that it's possible for certain kinds of recurrences to alter your program's security guarantees. As a test, how would you mitigate the DAO attack in Viper?
Finally, I fear that these flaws may be inherent to Ethereum, meaning that any language on Ethereum inherits these flaws structurally and cannot mitigate them. In that case, maybe it's time to design a capability-safe Ethereum!
I haven't looked into Viper and how it might address these matters.