You can say that about any service--what if Crashplan's data center caught fire tomorrow? Tarsnap might be run by a single guy (I think?) but saying S3 is "more" or "less" reliable than any other private company isn't a great comparison. In any case a massive company like Amazon is the most likely to be reliable in these cases, I imagine.
My earlier point was that I think data is stored on Colin Percival's S3 account (he is the creator of Tarsnap) and therefore you might lose access to the data (if he couldn't pay the bills or got hit by a bus) even though S3 itself is fine.
I use Arq to backup files on my MacBook - it saves them to my S3 bucket, so even if everyone working on Arq dies (I sincerely hope they don't!) and their servers all explode, my backups are still intact on S3 - which is less likely to go down permanently and lose all my files than Tarsnap (not that that is particularly likely either!).
There are definitely tiers of reliability. An external drive drive is easily lost or damaged. A backup service can take individual drive/server losses but that might be the limit. S3 can lose an entire data center - the only real risk comes from software bugs.
