I've never used the wireless units, as you said I've always heard they don't last/work long. We used to use some higher quality Omega/Monnit weather stuff for field monitoring that was wireless but you needed to have their receiver/software for it etc. There are lots of wifi "tags" now for temp/humidity but not sure any of them would function in a safe.
That said I've used these putting one in each safe and also in the room it's in. It's mostly just curiosity, to see how different the safes are from the room itself. It is interesting to see how two different safes in the same room and with the same dehumidifier setup differ in interior humidity as much as 3-5% but both still stay lower than the room by a couple points. I think the bigger deal is that you have some warming source in the safe that basically is stimulating airflow.
I can say that while cheap all of them have been within 2-3% of each other and within that of my kestrel, and much more accurate than my dehumidifier is, which can be off 10-15%. They also don't seem to vary much whenever I open the safes, even when the room does which leads me to believe the humidity in the safe is much more consistent compared to the swings in the room. So I don't really worry about it needing to monitor it wirelessly. That said I do have a dehumidifier in the room to ensure it never gets above a maximum humidity too.
https://www.amazon.com/AcuRite-Humi...=UTF8&qid=1550017493&sr=8-4&keywords=humidity
I also use an EvaDry 333 in each safe I recharge whenever it indicates it, which is usually 3-4 weeks, they are pretty cheap and seem to do the job well. Costco used to have 2 packs of them for about $20.
Also remember that a lot of people think you need it super dry in a safe, which isn't true at all, if I remember right the National Firearm Museum shoots for 70 degrees and 55% humidity. My guess is they are concerned about wood drying out/cracking if they go much lower. I'm sure if you had no wood guns you could go much lower.
