With many new ideas, the early folks love the benefits and aren't put off by the challenges. With ZTNA, after doing quite a few deployments myself, I can say that the biggest challenges are operational. Nothing will piss off developers more than having had access to a resource one day, lose it unexpectedly, and then not know who to track down to get it back. Or, users hating on their VPN, want something else, and then that something else (often just another VPN provider) works differently and causes them disruptions. ZTNA is a long journey, not a quick fix.