I've also got a 128G Sandisk microSD about 8 month ago, formatted it with BTRFS. It works nicely. Previous 64G AData (or was it SiliconePower?) died after 2 months.

bruce-one: I'd strongly advice against adding SD to the same pool as root fs. AFAIK to use additional space you will have to use RAID0, so if SD dies or isn't available at boot, you can end with corrupt filesystem. What's worse, "factory" home/root filesystems are stored as btrfs subvolumes, so they can get corrupted too, leaving you with unrecoverable system.

IDK what recovery mode uses for root filesystem. If it doesn't depend on those "factory" subvolumes, you may still be able to recover by restoring those subvolumes or the whole partition if you backed them up beforehand. Jolla won't provide you with images because of licensing restrictions.

Running 'balance' operation in this setup is especially dangerous since it'll most likely move some parts of 'factory' files to your SD to achive nice RAID0-like performance. In newer versions of Sailfish balance may run automatically if free space is too low.

So I'd advice to make sure that you have all filesystem/block dev backups and recovery mode uses some other partition, or be ready to ship phone back to Jolla for reflashing...