We were offered the same sort of thing but with Banco Popular. The incentive of subrogating appeared worthwhile until I actually found out how much a mortgage was going to cost compared with another lender. Their spanish interest rate was a lot higher for the first year ( 5%), so the savings were wiped out, and they could not offer an interest only loan, only a repayment loan. Another issue was with the accompanying current account which I had to take. The bank charges 6 euros per month for admin, even though I do not have anything else set up on it.
I have now changed to Halifax Hispania. I have an interest only loan on 1.8%, with a setting up fee of 1000Eu. The only other cost is a valuation fee of 350Eu. Also, the fee for the current account is fixed at 25Eu yr, plus you have an english speaking staff.
There maybe better deals about if you try hard enough, but I was happy with the Halifax.
Good luck