For some of us it's a financial issue. I already own a Raspi 4, but don't have money lying around to get a decent mini server (e.g. acceptable performance paired with low power consumption and no fan noise).

I still manage to run a few Docker containers on top of OMV, but need to be mindful of the load:

• Jellyfin (no transcoding)
• Immich (workers set to minimum)
• Backrest (restic frontend)
• Duplicati (phasing out)
• Heimdall
• changedetection.io
• Tailscale sidecar containers

But yes, when running a bigger backup job, I pause Immich indexing or shut down Jellyfin, just in case

Jeff Geerling ran some benchmarks on USB storage options in 2020, including a pen drive SSD: https://www.jeffgeerling.com/blog/2020/fastest-usb-storage-options-raspberry-pi

I also use those, but (shame on me) I've never tested the backups. Have you ever restored such an image backup?