If IF synonomyzes something, they are usually right. Often things that I consider synonymous are left separate. They appear to be a bit conservative with their synonymizing.

Why do you think that species listed as synonyms on IF are more reliable than species split into separate taxa according to IF?

Not all mycologists agree on taxonomic concepts, so sites like IF and MycoBank have to make a decision based on the information available. I would say that usually IF is correct because they attempt to reflect the current consensus, but some times the leading or current opinion on the nomenclature turns out to be wrong.

Of course, you can not rely on any single source.

Just because IF lists them as separate taxa doesn’t mean that they aren’t synonymous. If IF lists them as synonymous then they almost certainly are, but they are a mix of somewhat conservative and sometimes behind wrt synonymizing taxa.

Index Fungorum lists Cordyceps bassiana and Beauveria bassiana as two separate taxa, the former designated as anamorphic. If Cordyceps bassiana were the accompanying teleomorph, one would probably expect to see a sufficiently different looking fungus, but this is a dead ringer for B. bassiana, the anamorph. My initial suspicion was that the two were synonymous, B. bassiana being the current and preferred name, but I.F. refutes this.