Of course, the usefulness of a building savings contract depends on individual assumptions. The longer the term and the higher the interest rate assumed after the fixed interest period of the annuity loan, the more sensible the building savings contract. That is undisputed.
I made assumptions for myself personally at the time of the loan agreement (2012). Even back then, the general assumption was that interest rates could not go any lower, and of course I also assumed rising interest rates after the fixed interest period for my calculations. My calculations showed that a 20 or 30-year loan does not pay off for us, let alone a building savings contract. So far, I have been proven right. Especially since we make regular special repayments. In hindsight, a 10-year (or even better, a 5-year) fixed interest period would probably have paid off for us instead of a 15-year one. But since our financing is not overly tight, according to my calculations, even a strong interest rate increase after 15 years would not have completely derailed us. By now, I hope that after 15 years we will be almost debt-free. That was not foreseeable in 2012.
You need to get various offers for yourself and then calculate different scenarios. You will probably then find that long fixed interest periods and the building savings contract only pay off if you assume strongly rising interest rates, and then you have to judge for yourself how likely you think falling, stable, slightly rising, and strongly rising interest rates are, and whether you can cope with a strong interest rate increase if necessary. The calculations can be done for you if needed, but you have to make the assumptions. Your fully amortizing loan is of course the all-round carefree loan, but that of course costs.
Due to the ECB's target setting regarding inflation and also the debt ratio of the EU countries, I personally consider strongly rising interest rates >5% unlikely. That would lead to a chain reaction causing a major crash. Such a strong interest rate increase will therefore only occur if all the tools of the ECB and all European countries have already been exhausted, and I do not even want to imagine what would come then. However, I also never would have thought negative interest rates were possible. So you can always be surprised.
All this is my personal layman's opinion.