The problem isn't that Linux can't do hard rt or not but rather the particulars of the system. RTOS' are typically made with very particular constraints in mind and a desktop is pretty much at the opposite end of simple controllers you seem them on.Even with a "realtime" kernel, Linux is not hard realtime and neither is JACK. On my system, a dropout means the "xruns" count in qjackctl increments and I mutter obscenities and remember the timestamp so I can go back and check for clicks when I'm done. No server death unless you want it.
An interesting place to see read about various systems latencies is OSDAL. They'll run their systems for a year at a time and chart the latencies. Even under load, the jitter is surprisingly low on these general purpose machines.