The pure-play Bitcoin miner still faces some long-term challenges.Riot Platforms (RIOT -3.56%) became one of the world's largest Bitcoin (BTC 0.15%) miners during the past few years, but it's generated disappointing returns for its long-term investors. Riot actually used to be Bioptix, a developer of veterinary products, before it abruptly abandoned that business model (and name) nearly seven years ago.On Oct. 4, 2017, Bioptix rebranded itself as Riot and declared it would become a Bitcoin miner. It subsequ ...