Android unusable, were my expectations too high?

[g_t_j]

There are tons of counterfeits Sandisk cards out there, so unless you bought it from a reputable seller, chances are you have a fake one. In the past I even bought a Sandisk ultra from Amazon uk which I later verified it was fake with the help of application ''SD insight'' !

I now solely use ''SanDisk 16GB U3 Extremes'' which work flawlessly while the community recommends Samsung EVO+. I think whichever you buy you will be on the safe side.

The point here is that you should not worry about your Pine64's performance on Android. The experience you had so far has certainly nothing to do with the board's normal performance.

It's a genuine SanDisk according to that app, but comes up as a 63GB SL64G rather than a 64GB SU64G like my others? Maybe SanDisk themselves have started selling inferior cards in their Ultra line. I tried it in Crystal Disk Mark (on 2 different PCs) and the random read/write results are terrible, much worse than even the PNY card mentioned in the thread benpope81 linked above. I'll go and buy an EVO+, thanks

As long as this app finds it genuine, it is as it reads the serial and barcode by the manufacturer. 
Obviously you got a faulty one. See if you can return it for a new one.
The EVO+ is considered to be value for money with the best sequential write speeds.

Enjoy!