Although they may not have actual stats, tablo is marketed as a consumer product to the general public with little to minimal technical knowledge. Most likely an extremely accurate educated guess “very few users upgrade their HDD”.
Inversely, how do you know customers are leaving their tablos because they bought too small drive in the beginning and can’t easily upgrade to a larger one?
Many of users have found it fairly easy, wrote step by step posts for others. Difficulty level is subjective there are those who’ve multiple times changed. Depends how important it is.
Sadly, this don’t officially include internal drives 
If you replace your Tablo DVR the process below will allow you to migrate your existing recordings stored on an external disk (devices using internal storage, SATA storage or Cloud storage are not supported) and schedule data to your new unit.
I’m not sure you’ll find external storage which is not SATA, so that’s a bit flubbed up.
After what happened in 2020, anything could happen! At one time, tablo said the thing holding them back was a requirement needed root access:
but back-uptablo.sh could assist
more than 
0.02 – they go virtually anywhere, as in one piece. Tablo is pseudo-propritary, it/when something happens to it… if you have your video library archived you’re pretty much SOL. Exported videos can be backed up. (though there is a backup script for tablo) If the drives does fail there are tools to possibly recover data.