Nothing is guaranteed about delivery with LoRaWAN at the comms level and there is absolutely no guarantee about timescales of delivery up or down. So any confirmations at firmware / application level may take many minutes if not longer. For TTN there is a fair use policy of 10 downlinks a day.
Every time a gateway transmits an ACK or a downlink, it is unable to hear any uplinks at all, so any uplinks from devices that only in range of that gateway are lost.
If you need this data to be transmitted frequently, you should also be aware of the TTN fair use policy for uplinks and your local legal restrictions on duty cycle.
As for your application @Erikjan, I can’t begin to describe how totally inappropriate your application is, even if you are using TTI. If you are using TTN without an SLA then all bets are off.
If you don’t know the LoRaWAN MAC or this implementation of it that well, maybe you shouldn’t be using it for this application.
I’d have to assume you have a LoRaWAN gateway on LTE on each & every appliance - so you can ensure that the system works regardless of which vehicles are at scene. And then you run in to the issue of LTE coverage.
I type this as someone who builds safety critical systems for security and as a first responder who would never want LoRaWAN to be part of my safety net.
If @Erikjan wants to cut to the chase, a module like the NRF24L01 would be far more appropriate if you are trying for a proof of concept but I’m not necessarily recommending that for production.
For the purposes of enlightenment and discussion I’ll leave this topic open to discuss LoRaWAN suitability for safety critical sensing applications.