If you make special repayments, you no longer have to pay interest on the repaid portion over the remaining term. With the building savings contract, you save at zero interest, and after the loan for which you have been paying interest the whole time expires, you repay it with the building savings contract for which you have received almost no interest. Interest loss.
Putting aside other disadvantages of the building savings contract ([Auszahlung nur nach Erreichung der Zuteilungsreife, Abschlussgebühren etc.]).
If you reduce your loan as much as possible within the fixed interest period through special repayments, you make yourself more independent of future interest rate developments because the remaining debt is accordingly low and even with sharply rising interest rates the burden does not necessarily increase.
Special repayments are only realistically feasible if a positive income development is foreseeable or if surpluses are generated all along, so that you can afford them. Fundamentally, in my opinion, this applies even more to the building savings contract, because the special repayment is effective even if less is repaid than originally planned.
With the building savings contract, you might not reach the allocation maturity and have paid into it, the fixed interest period runs out, and still you don't get access to the fresh (favorable?) money from the building savings contract.
In my opinion, not really a gain in security.
