Signal is the world’s most widely used truly private messaging app, and our cryptographic technologies provide extra layers of privacy beyond the Signal app itself. Since launching in 2013, the Signal Protocol—our end-to-end encryption technology—has become the de facto standard for private commu...
All of those. Essentially you would have to go out of your way looking specifically for incompatible clients.
And “incompatible clients” is simply the natural state of any technology that’s been around long-enough. The only way Signal fends itself from this is by mandating its own client and version (and banning anything else, technically or from its ToS) which is terrible for a bunch of reasons (you must agree with Signal’s direction and whatever features they might decide to add and remove for your own good, you cannot use Signal on devices/platforms that Signal has no resources/interest to support, etc). If Session is in any way open, and assuming it ever becomes successful, it will face the same challenge (just like Matrix does).