I may need to add on to the house to have enough devices to be reliable.
Digger -
I really think you should look at your house to see what's causing so much interference rather than trying to compensate by adding more and more Insteon devices.
Switch reliability and failure rate is one thing, but I very rarely hear anyone talking about signal reliability - and when I do, it almost always ends up being some sort of interference by an appliance, UPS, PC, fluorescent light, etc... I don't think it's a common issue, at least not to the extent you're experiencing.
Obviously adding more Insteon devices will probably help, but I think in the long run it would be better for you to filter out the source of your interference.
My Insteon switches are almost 100% reliable switch to switch. I am having some issues controlling via CQC, but that's a whole other story...