I got cross-vaccinated here in Germany due to being young and getting Astra Zenica as a first dosis. I still had the choice between a second dose of Biontech and AZ, so I read quite a bit about it at the time, and for example one of the leading Virologists in Germany basically said he wouldn't know why it shouldn't work. Basically, the data suggests that there isn't a suprising mechanism that does lead to less protection.