I personally use Arq against Google Cloud Storage. I have around 1TB, and I use it to back up several external drives in addition to my MacBook. (Arq is nice in that it backs up the drive if it's connected, but won't prune olds backups if it's not, unlike services such as Backblaze.)
I think I pay around $6-7/mo. I use a coldline bucket, which is the cheapest kind of bucket. Restoring is super fast (unlike Amazon Glacier, which I used previously) and not too expensive.
I wouldn't risk my amazon account on it, personally.
There are plenty of people who are storing absurd amounts of data [1]. If Amazon were cracking down on this, you'd hear it in /r/DataHoarder first.
The ToS [2] don't mention any size limitations. In fact, it says you can store pretty much any file as long as the data doesn't violate any laws, including copyright.
The only really worrying part is this:
3.2 Usage Restrictions and Limits. The Services are offered
in the United States. We may restrict access from
other locations. There may be limits on the types of
content you can store and share using the Services,
such as file types we do not support, and on the
number or type of devices you can use to access the
Services. We may impose other restrictions on use
of the Services.
[2] https://www.amazon.com/gp/help/customer/display.html/?nodeId...
I think Borg would be a great solution for my use at home where I have a NAS on my LAN to provide local storage for the repository. But while traveling the world with my MacBook it doesn't cut it.
Arq looks good from that angle but I think would quickly cause my costs to overrun me. Having good backups is what gives me the confidence as a photographer to overcome my inner pack rat. Without good backups I'd never be able to ruthlessly delete images from my collection that don't exceed "okay" into "great". But because I know my images are backed up effectively forever I can really be honest in my art.