Why is it difficult? I run a medium size website, with a custom threaded comment system. It receives 30,000 new comments a day on average. There has been zero issues, it's a simple comment table in the database with an index, and some standard queries. It's something you could literally code up in an afternoon.
Now, if you receive a hundred million comments a day, it'll be a little more complicated, but those sites are few and far between. At that point, you most likely have a budget for better hardware and specialized staff.
Now, if you receive a hundred million comments a day, it'll be a little more complicated, but those sites are few and far between. At that point, you most likely have a budget for better hardware and specialized staff.