Different interest rates are one thing, but the other conditions with the 124 are also rather off-putting...
depends:
If you don’t plan any special repayments (or possibly only up to the amount X, which is used for the other loans), this disadvantage would already be eliminated. For financing €360,000 (as far as I still remember correctly), you first need to have €18,000 (5%) per year available for special repayments.
You overcome the interest-free period by paying the first invoices from this loan.
The only disadvantage that I think might apply is – as described by – possibly a better interest rate at the house bank with reasonable creditworthiness.
Oh – I almost forgot one more "disadvantage": With KfW, the repayment portion is relatively high, resulting in an annuity between 6 and 7%. In tight financing situations, the bank loan might be more sensible. However, you pay off for a longer period.