With all due respect,
cash registers are for
cash. Digital currencies don't need those silly/bulky old machines
If you were thinking of accepting cash and digital currency, and wanted to do this for accounting purposes, I wouldn't use the register (itself) as the medium of aggregation.
I suspect that the concern has more to do with internal accounting, security, and sales tracking than the specific use of a "silly/bulky old machine".
When someone says "cash register", they typically mean "Point of Sale System".
As such, the "Point of Sale System" typically will need to be able to hold cash for those that continue to pay with cash, track credit and debit card transactions, and in this case track bitcoin transactions as well.
Since the system will need to continue to hold cash (at least until bitcoin completely replaces cash in society), it isn't really silly to call it a "cash register".
Regardless, I suspect the OP is asking about the difficulty in modifying a merchant's current transaction processing infrastructure to include bitcoin transactions along with all the other transaction types they already handle.