They are certainly cheaper.
One of the problems seen by some people with OEM batteries is variation of cell output, where one of the three cells loses output, and drags down the entire battery pack quicker. When that happens, a 20 minute battery becomes a 10 minute battery. I have read that DJI is replacing batteries like this under warranty.
But, considering these are probably 'seconds' - units that aren't quite first line, I'd expect this to happen more often. You might get lucky.
Of course, these COULD be first class batteries, made by young idealists running a business that eschews the profit motive and only wants to make great batteries at a great price, and pass the savings on the noble, deserving, Phantom owner like you.