I'm in the middle of replacing my insteon sensors with z-wave, but I also have quite a few Insteon switches with the I2 engine which are prone to spurious signals. I find that once in a while, all my I2 switches turn on or off ( a known problem). I think that the newer switches are programmed to ignore the all on insteon command, and the I2cs engine is generally better ad detecting corruption.
So with that in mind, would you recommend replacing those I2 switches with I2cs switches or with z-wave ones. I have around 150 devices in total, of which 33 are of the I2 verity. So either way a substantial cost for me.