That's either plain nonsense or trivially true of all css, depending on your definitions.
Flexbox really isn't all that confusing or hard, and it does more than table layouts ever did.
Separate flex-grow and flex-shrink are needed for the same reason there's a min-width and a max-width property. flex-shrink defines if the element is allowed to shrink, and how much it does when needed. flex-grow defines if the element is allowed to grow, and how much when needed. The reason they are unitless values is because they are used to calculate ratios.
I agree that there have been some dubious design decisions, but if you really need to do a table cell with a flex-flowing content you can always stick a wrapper div in the cell.
"you can always stick a wrapper div in the cell" Nice, tag soup forever.
That's the thing - nobody knows exactly how it works. Except few developers who actually tried to implement the spec.
Nobody knows why do we need separate flex-shrink and flex-grow. Nobody knows why these are properties and not units so they can be used not just for width/height but for padding, margin, border-width, top/left/bottom/right, size, etc.
Nobody knows why 'flex' is a value of 'display' property and not of some other property orthogonal to display like 'flow' or 'layout'. So we can write something like this:
At the moment there is no way to define flexible content flow inside table-cell's and list-item's as display:flex is mutually exclusive with display:table-cell.Really, flexbox is the worst CSS module ever published and accepted by W3C.