Classic UNA vs a headless NEO front end: how do they compare at scale?

A question I keep turning over while we plan this network: how does a classic UNA site, where PHP renders every page, compare with running UNA headless behind the NEO front end once membership gets large?

My understanding so far: the classic setup is simpler to run and every module works out of the box, but each page view goes through PHP and MySQL, so caching does most of the heavy lifting. With NEO, UNA mostly answers API calls and the Next.js side can cache public pages close to the visitor, which should help with traffic spikes. The trade-off is one more service to deploy and monitor, and real-time features still need something like WebSockets either way.

If you've run either setup past a few thousand active members, where did it start to strain first? The database, media storage, notifications?