RAID10 vs. RAID5 for data files
There are two types of activity to consider here: reads and writes.

As you correctly point out, MDF/NDF writes are done through the checkpoint process, and users shouldn't have their transaction time affected. 

Reads happen when the data that is needed is not yet in RAM (in the buffer cache). So ideally, user activity shouldn't be affected often. But when it is, the data can be fetched quite quickly whether RAID 10 or 5. The pain point for RAID 5 is with writes not reads.

But if you lose a disk - what then?

With RAID 10 there's no loss of performance. When a replacement disk is put in, the mirror is copied over it. With RAID 5, every read requires some maths to be done. And when a new disk comes, it has to read a lot and write a lot.

But RAID 5 is cheaper. 1TB of disk will get you 800GB of space (depending on your setup), but RAID 10 will only get you 500GB. Disk activity is slightly more expensive though.

It's "fine" to put data files (not logs) on RAID 5. But if disk is cheap enough, RAID 10 is better.