There was a Linux client I saw a while ago—Spring Mail or something like that?—which first downloaded your email from your provider onto their own servers, then your local client got them from their server. This additional cloud step is what I want to avoid.
Just keep in mind that sd cards can also stop working. I would encourage you to keep at least two separate copies, ideally one on a different medium than an SD card, and check regularly that you can still access the codes. A backup is only as useful as it is easy to restore.
If you have access to a Mac, use the Apple AAC encoder. It will give you better results than libfdk_aac, which itself is much better than the ffmpeg built-in aac encoder.
The author does a pretty good job of explaining the potential problems this technology could cause. Scroll down to Why Attestation Is Bad.
I have not, but from screenshots it seems only a minor reskin.