The usual caveats are things like "we can't build a car-sized one" or "we can't manufacture it cost-effectively" or "we thought we could improve it a lot but co

The usual caveats are things like "we can't build a car-sized one" or "we can't manufacture it cost-effectively" or "we thought we could improve it a lot but couldn't". New breakthrough every month! Shame that almost all of them don't pan out.