You usually build a chain of hashes of messages, not of the messages themselves. Meaning that you can delete the message, leaving evidence of the deletion in the form of a lonely hash in the chain.

Yes, but if uncensorability is a feature you want, there's nothing stopping you (other than the enormous cost[1]) from using the messages themselves.

[1]: Not necessarily a deal-breaker. For perspective, consider that a 10-word transatlantic telegram in 1860 cost the equivalent of $2600 in 2013 USD.

Another solution is to store a URL and a integer score (so you can rank companies). The longest URL I was able to get through CloudFlare recently was about two thousand characters but most will be much smaller. If someone wants the review taken down they can talk to whomever is hosting the content. Tweets, Medium articles, Facebook posts etc could hold the review content itself. Want to take down 5 unfavorable reviews, then write 5 letters to 5 different companies.

As another user noted, you can encode the message into the chain though.