This could definitely be an issue, but in my use case I think I'd find the perp pretty quickly and scare him off with my pitchfork :) . It's important to space out the messaging, I don't send more frequently than 1msg/minute, and messages are 8-12 bytes, and most sensors are 1 msg/hour.

In general LoRa (and LoRaWan) are designed to be very resilient against interference, but targeted jamming seems possible: https://arxiv.org/pdf/1712.02141.pdf

On the other hand, jamming is possible for most radio protocols. I'm pretty sure it's illegal though, even in the license free band.

And LoRaWan in standard mode is inherently unreliable (unconfirmed packages by default). Using it for "passive" logging is fine, using it for critical "active" remote control maybe less so.