Will I be able to complain if my microSDXC-Card doesn't work in the Jolla tablet as advertised by the manufacturer of the card? In terms of speed and lifetime?

Cards come pre-formated in a way that best matches the physical layout of the card memory. If you either re-format (or worse: re-partition) the card, you need to know all the necessary parameters. Else the allocation blocks of the file system will cross the boundaries of the cards' physical blocks, resulting in lower speed and reduced lifetime because of unnecessary read-/write cycles. It's quite tricky to find out the physical parameters of a given card, much of this is guesswork.

Now if the tablet includes a one-size-fits-all unisex card formating tool, chances are that the card you bought because SanDisk claimed it'd reach 48MB/s is in fact much slower once re-formated to ext4 in the tablet. Where do you get support now? Jolla? SanDisk? The SD Association? Wo will make your card reach 48MB/s again? Who will take it back and return the money?