I think they are really trustable and reliable.
They are a German bank and not are only a bank, they also do BaaS (Bank as a Service), means that they form white label banks for companies which want an own bank and they have some famous clients like for example O2 Bank from the trademark O2 of the company Telefonica.
They are secured by the EDB (Einlagensicherung der Banken, which means something like "deposit guarantee), which means that they are at least secured for 100.000€, however Germany even offers to pay back sums until 500.000€, this takes a lot longer, but they do it in most cases. https://www.edb-banken.de/banks/abfrage-fidor-bank-ag/
@Admin Business account for GmbH should not be a problem at Fidor, they accept GmbHs @daxbr yes, 100k is standard for all banks in Germany. However Germany also offer paybacks until 500k.