The big problem with the E310 is that after you've prototyped something and want to design a custom board, you've got a much more difficult task ahead of you. The AVR needs a significantly fewer number of support chips, as it has onboard flash, has pretty large voltage tolerances, has adcs, etc. It gets even murkier when you start considering boards from other companies like STMicro, which have Cortex-M7s, are competitive in performance and have boards that go for half the cost of a hifive1. It's interesting if you want to poke around with Risc-V for the purpose of poking around, but i wouldn't want to start a serious project based on it at the current time.