From a technical perspective, I think what they built is great, but at rates around a penny per message, how could I make a profit? A single large customer such as Uber makes sense, but if your business success is based on a unicorn, then good luck to you.
I really think that Twillio is first to market because of the inept management of telcos, but how do you compete when they figure it out? Further, the rates per message drop dramatically on a contract for volume, especially in the wholesale market.
Then of course there is O365 Microsoft Skype for business, and I expect that all of these capabilities will be available with their UCWA api.