I'm not qualified to understand the context in which Weiss gave their ratings, and they didn't release much to the public about their methods. However I suspect they don't understand the crypto market as well as they maybe should, as evidenced by the fact they considered 100k unique users from Korea a DDoS attack. Given the popularity of crypto in Korea I can see this just being people trying to get the report they heard rumors about. (I've had web-sites I've administered for regional rock bands get as much traffic when they released a new album.) Maybe being DDoS'd sounds better then "ooops we couldn't handle the server load" in their PR material.
That said, and to answer your question, I'd give Bitcoin a C+ too, for high transaction costs and slow transactions.