[Xperia Z3 Compact Original Android Development] Nazo:
Well, if a different release has different results it can't be 100% hardware (specifically not the antenna since that's one piece of hardware that wouldn't know any difference in different ROMs.) However, I can say that I too have had zero troubles with any semi-recent release and Bluetooth (I think a long time ago I had an issue with this line where for some reason it would boot up with the Bluetooth off and I'd have to manually turn it back on after every reboot. Otherwise it stayed on and worked fine from that point on.) My guess is there's something more finnicky involved such as maybe there are different Bluetooth chipsets and they might have different results depending. (Also, as I recall the Z3c has two revisions -- a D5803 and a D5833, right? Apparently the only difference is officially the LTE specs used, but it's not impossible the chipsets are just slightly different perhaps?) As for GPS, that is even trickier. GPS on a cellphone often actually uses cell towers and even WiFi for triangulation (apparently using an actual true GPS signal uses a lot more power?) and there's probably a lot that can affect this. Perhaps even APN settings could be an issue? I just found myself with no data access in a random place then two people who should have been on the same network had it and I realized my APN was set wrong (I just didn't notice since it worked everywhere else up to now.) Otherwise I have no clue.

Anyway, all I can tell you is my D5803 has had very few troubles with any of the ROMs I've tried from this thread. I'm using Bluetooth with that last release from the OP quite a lot. For the longest time I've been using it with a Pebble smartwatch and a car head unit with Bluetooth reception built in (both hands free and music playback.) One thing of note: some things like to take over your "audio app." For instance, Youtube is kind of bad about this. If you have background data disabled, this can result in silence (or maybe an error?) when you hit play on a head unit if you recently ran Youtube it seems. Drives me absolutely batty when I find it's trying to play a Youtube video instead of using my official default audio player. This is not a ROM-specific issue, this is an Android handling thing (namely it's Youtube's fault for taking over -- particularly without permission.) I have to pull over and open my normal music playing app (I don't hit play until I'm out of my driveway and my phone does not leave my pocket while I'm driving,) then everything works fine. So if your problem was anything like that make sure it's actually using the right app. Never once while music was playing has it ever dropped or anything. I've had some issues with my new smartwatch (Ticwatch E,) but that was in setup and once I finally got that right it has been completely stable on this one.

We're all going to have to replace our Z3Cs eventually though. It pains me to say this. My biggest hope is just that I can hold out until there's something of a similar capability (including reasonable third party support) that does NOT have a freaking glass back. (Seriously Sony, what in the heck were you thinking??? Yes it's shiny and smooth. So is a coated plastic or a metal!) Haha, it may be a while. So yeah, I'm loving this because of its clean reliability. We really shouldn't have been able to hold out as long as we have and shouldn't be able to run 7.1 or even 8.x on this thing, so I think we should be grateful we can.