there is no physical different between SDXC and SDHC card, they absolutely speak the same protocol (MMC).
the only difference is a logical/marketing one :
- SDHC are <= 32 GB and are sold pre-formatted with FAT32.
- SDXC are > 32 GB and are sold pre-formatted with exFAT (which is patented and cannot be provided by Jolla).

(that's different from plain SD cards which use a slightly older version of the protocol, that can only address card up to 1GB, or up to 4GB with a few hacks. the difference betwen SD and SDHC/SDXC is a bit similar to the introduction of LBA and LBA48 in harddisks).

this current situation is an entirely different problem : the card and the chipset are negociating a clock frequency to talk to each other (MMC uses a separate clock trace) - at least that's what it seems given the differences in dmesg between my card and the problematic card.

It currently seems that if the kernel is 3.10.84, this step fails (bug seems to be known in Ubuntu communities).
if the kernel is more recently (probably like the one packaged in Android 7.1.1 Nougat by Sony), this bug seems patched.
At least, that's what looks the most likely given the problems (card always works in stock android system and fails after Sailfish X flashing).

It just randomly happened the this card happens to be a 128GB one and also happens to have the frequency error.

But other people on TJC have successfully used a different models (the 256 Evo+ from Samsung).