With NVMe SSDs? What makes RAID unviable in that environment?

This depends, like all things.

When you say RAID, what level? Software-raid or hardware raid? What controller?

Let's take best-case:

RAID10, small enough (but many) NVMe drives and an LVM/Software RAID like ZFS, which is data aware so only rebuilds actual data: rebuilds will degrade performance enough potentially that your application can become unavailable if your IOPS are 70%+ of maximum.

That's an ideal scenario, if you use hardware raid which is not data-aware then your rebuild times depend entirely on the size of the drive being rebuilt and it can punish IOPs even more during the rebuild. But it will affect your CPU less.

There's no panacea. Most people opt for higher latency distributed storage where the RAID is spread across an enormous amount of drives, which makes rebuilds much less painful.

What I used to do was swap machines over from the one with failing disks to a live spare (slave in the old frowned upon terminology), do the maintenance and then replicate from the now live spare back if I had confidence it was all good.

Yes it’s costly having the hardware to do that as it mostly meant multiple machines as I always wanted to be able to rebuild one whilst having at least two machines online.

If you are doing this with your own hardware it is still less costly than cloud even if it mostly sits idle.

Cloud is approx 5x sticker cost for compute if its sustained.

Your discounts may vary, rue the day those discounts are taken away because we are all sufficiently locked in.