I can't think of a "highly negative" report off-hand. I'm not sure how I'd word things now that I think of it. It very well might take a long time for me to write.
I've had mediocre service before and even one or two actions that were pretty bad I've had to write about. But, in those cases, I usually use the language:
"The overall experience was mixed." From there I go into the positives and negatives.
If possible/appropriate, I'll even use Steven K. Scott's "sandwich method" for delivering criticism. He says in decades of doing business (he's a multi-millionaire and record-setting direct marketer, known for his commercials/informercials generating billions in sales) he's never once seen it fail. Basically, he says that when delivering criticism to someone, you can help them better accept it by first saying something positive about the person (but, make sure it is true and don't just make up something here). Then, deliver your criticism. Afterwards, say something positive again to close the conversation. The positives are the "buns," while the criticism is the "meat" in the sandwich. By cushioning your criticism this way, it helps the person take it more easily. They'll understand you see some value in them or are really on their side (not just out to tear them down) and will be more likely to accept and learn from your criticism.
If you've ever watched Shark Tank, I think a lot of the sharks use this technique (or some similar approach) when rejecting pitches. It really can be hard to criticism someone and/or turn them down.