@arubislander
Hi!
I also have a Oneplus 5T. Latest UBP.
Without sim card (and thus all mobile data to deal with), the battery is ok : lost 10% after 24h idle.
I understand that "battery saving" cuts mobile data when wifi is available.
Now, the battery will deplete in no time when only mobile data is available.
On my 3rd day with LOS, battery is at 31%.
Though I've spent time away from my wifi servers.
Conclusion (sort of, I'm no dev!!!) : management of sim card by UBP is a battery monger. It's a no-go for me as I often spend time away from my home/work networks.