But that doesn't mean we should sit here and say "nope, can't be done".
People were skeptical when planes first flew, when it was attempted to put people on the moon, and so on.
Jeez, people were skeptical when SpaceX said they could land a first stage on a barge at sea, and posted endlessly about how it couldn't be done.
Don't be one of those people.
Even if they "fail" to make all of this work and get something to Alpha Centauri, they're still going to vastly expand mankind's knowledge of a lot of difficult space-related technologies, which is a huge win.
This is hacker news, where people are trying to do difficult things to change the world. That's the whole point.