and from a really simplistic point of view, it is still SIP, just another implementation of it with “more better” features. Nearly, but no not all, things I have tried work with it just fine. Designed to or not.
I’d also say it’s a lack of understanding of SIP, as chan_pjsip doesn’t do magical things on your behalf - an example being that it speaks/configures in URIs, which can throw people off.