As a result we use MySQL w/ memcached, although we are considering a swap to redis for the caching layer.
That is definitely not ok. I'd be really pissed as a user if I wrote a huge comment and it suddenly disappeared.
Note the tradeoff doesn't make sense as soon as you're operating at a meaningful scale. A small likelihood of failure at small scale translates to "I expect a failure a million years from now", whereas at large scale it's more like "a month from now". Accepting the same percent risk of data loss in the former case might be OK, but in the latter case is irresponsible. Provided whatever you're storing is not transient data.