"But it is encountering major network bottlenecks, causing delays and high transactions costs."
"Despite intense ongoing efforts that are achieving some initial success, bitcoin has no immediate mechanism for promptly upgrading its software code."
Bitcoin, the largest digital currency by market capitalisation, traded near US$10,955 ($14,901) today, according to CoinDesk. Ethereum, the second largest by market capitalisation, traded near US$1,019, according to CoinMarketCap.
None of the 74 cryptocurrencies measured received an "excellent" A rating. B-rated ethereum and digital currency EOS had the highest ratings.
According to CNBC, Weiss' tough take is a trademark of the 47-year-old independent financial ratings agency.
Reports from Barron's and The New York Times have noted Weiss' lack of A ratings in coverage of insurance stocks, mutual funds and other securities.
Despite the tough ratings, Martin Weiss, who founded the Florida-based company, insisted cryptocurrencies have a "bright future", following the announcement of the ratings.
"Despite extreme price volatility, cryptocurrencies have a bright future and the potential to deliver unusually large profits to investors," Dr Weiss told the Express.
"However, the market is hectic and confusing for investors. They need the clarity that only robust, impartial ratings can provide."
Weiss told CNBC's "Squawk Box" that the agency uses a computer model to rate cryptocurrencies based on four metrics: risk, reward, technology, and fundamental aspects of adoption and security.
"You can have a great technology index — all the pieces are in place for a very nice, strong currency — but unless it's actually reality-tested in the marketplace, it may not succeed, and that's what this last, fourth index tests," he said.
The release of the results appeared to be hotly anticipated by cryptocurrency investors.
Weiss reported a huge surge in visitors to the site in the wake of the publication. This included thousands from Europe and Asia.
"Since our customers are almost entirely in the US, we rarely get more than small amounts of traffic from Asia," Weiss told the Express.
"But last night, we had more than 100,000 unique visitors in a matter of hours, almost entirely from Asia, until early this morning, when more of the traffic rolled over to Europe."
"To handle the volume, our staff instantly expanded its server capacity on the cloud many times over."
Ari Paul, chief investment officer at cryptocurrency investment firm BlockTower Capital, said in an email: "Weiss' cryptocurrency ratings are a great example of the ongoing institutionalisation of the cryptocurrency industry and a healthy addition."
Ripple, the third largest digital currency by market capitalisation, received a C rating, according to the release. Ripple traded near US$1.37 today, according to CoinMarketCap.
Other well-known cryptocurrencies such as Dogecoin received a C rating, while Litecoin, the seventh largest digital currency by market capitalisation, was given a C+ rating.
The equal highest rated cryptocurrency, EOS, traded near US$14.52 today, according to CoinMarketCap.
