I've owned well over a dozen of those cards and they DO fail occasionally. While they don't necessary get "slower" over time, I have had some cards that failed intermittently.
I recently had a 128GB card fail in my Hero4 camera. It would record for a while...over an hour...then all of the sudden I would see the dreaded "SD CARD" failure message on the LCD display. After this happened like 3 times I sent it back. SanDisk replaced it with an upgraded A2 card. They cited that they don't recommend the A1 cards for Hero4 cameras, but literally nowhere on their website does it say anything about not using the A1 cards in a Hero4. At any rate, I stuck the replacement card in one of my 720p security cameras (that records 24/7) and put an Extreme Pro (A3) in my Hero4.
I think I've had 3 SanDisk failures - not all Extreme Pro - but that's pretty much the only card I buy any more these days. Because they DO fail, I ALWAYS have a spare. And I regularly rotate the cards so I'm not caught with my pants down. SanDisk is very good about replacing their bad cards.
And, of course, I don't have to tell you to beware of counterfeit SanDisk cards. I swear, I think the counterfeit market is larger than the actual SanDisk market.
I recently purchased six 128GB SanDisk MicroSD cards on Amazon for $43/each. Any less than that, and I would expect counterfeiting. And so you know, I HAVE purchased counterfeit cards on both eBay and Amazon. I was refunded both times.
I was running 64GB in everything and recently upgraded all 4 of my security cameras and drones to 128GB. So far, so good.
Thanks for your reply I've used the same four cards now for three years, and a bucket load of 32s and 16s that I just keep when I purchase something with a sd card in. The cards just seem to run on with no problems. Maybe I'll get a couple of 128GB for my arsenal.