So definitive answer you are looking for:
Bad news first:
I am by law not obligated to give any refunds for customer specific equipment like the bitburners as stated in my AGB (Terms of Service).
Comment on this: The Bitburner is, to be honest, not an individually designed, non otherwise sellable product and this might not hold in german courts. Prepare some money for the crazies...
It is a custom made PCB....
Not really. It is an assembly with a prefinished product. Last time I heard such a discussion and it went to court it was specifically important that the product is specifically made for the client, that is after his wishes, and cannot be sold without loss to anyone else.
As an example, if you had used it for, lets say, a computer, the computer would have the same warranty and rules as any bought computer, even if it was specifically assembled for you. BUT: If you had your NAME engraved on it on the side, at that point it became an individually constructed item for the customer.
As we say: In German Courts and the high sea, you are in the hands of chance.