You may be interested in reading this: https://uphillathlete.com/why-we-stopped-relying-on-hrv-apps/ In my fairly extensive experience using HRV testing apps with several professional athletes I finally abandoned the use.
EPOC is the brain child of Heiki Rusko who is one of the grand old men of exercise science. He is one of the leaders in HRV etc. So, who am I to dispute his wisdom. From my understanding EPOC is also a HR based training load measurement. It relies upon the HR zones you tell the device.
For real time intensity monitoring for endurance athletes in variable terrain sports like XC skiing HR is still the best metric. For post exercise training load monitoring, Training Peaks Performance Management Chart tops anything I have ever used in 30 some years of looking.