Comment by jcranmer

I'm hardly a connoisseur of DER implementations, but my understanding is that there are two main problems with DER. The first is that the format isn't really parseable without using a schema, unlike (say) XML or JSON. This means your generic DER parser needs to have an ASN.1 schema passed into it to parse the DER, and this leads to the second problem, which is that this ends up being complex enough that basically every attempt to do so is full of memory safety issues.