Expert despite TÃœV approval?

In this case, I can only speak for the automotive industry, and here, courtesy reports are part of everyday life. No expert in the world will deliver a report that contradicts the interests of their (regular) client who pays the bills. That is the way things are, and there is nothing to interpret otherwise, no matter how you may argue. Neutrality can sometimes be expected only from a court report, and even then, one cannot often be sure...

... No expert in the world will deliver a report that contradicts the interests of their (regular) client, who pays for the service........Neutrality can sometimes only be expected in a court expert opinion, and even there one can often not be sure.

Yes, unfortunately this is often the case, regardless of the industry! The only protection here is to hire an external expert oneself (construction supervision). It is usually sufficient to focus on critical key points. This also keeps costs within limits. Example: In a new building, a stone X with 0.09 W/m²K (KfW proof) was planned. On the construction site, however, a stone Y with 0.13 W/m²K was delivered and installed! Long-lasting consequences! Coincidence, oversight, or intent?

best regards