Everybody talks about the engineering challenges, which have been beaten to death. Elon Musk has a pretty respectable track record of making things that seem technologically infeasible and making them happen. He's not in charge anymore, but I trust that he wouldn't put his name on something that's not possible. Even if it wasn't possible, I'm not the person to make that critique.
The bigger challenge, IMO, is economic. High speed trains make a lot of money, but their revenue has little to do with competing with airlines over significant distances between large cities, which is what the Hyperloop is attempting to supersede. The financial success of high speed trains has much more to do with their ability to rapidly start and stop in lots of intermediate urban areas within a corridor. I know this goes against the popular narrative about why high speed trains are popular (OMG they can compete with air travel!!!), but the narrative is wrong.
The revenue of HSR has a lot more to do with the number of cities that it services than the end to end speed. If you look at ticket revenues on successful HSR lines, terminus to terminus tickets typically make up possible market share (trains, planes, cars) between terminus cities, it would still be less than the total revenue for that line, due to the additional cities it serves. Even with the gradual growth in power levels for high speed trains, with some now capable of 360kph, end to end speeds have basically gone unchanged. What has changed with more powerful trains? The lines have more stops now...they serve more cities, with the higher power levels used for faster acceleration and deceleration. More cities means more revenue, whereas higher speeds have already hit strongly diminished returns.
Maybe with Hyperloop being really fast could grow the market and monopolize market share between Alpha cities, but that would still be a stretch if the goal is economic viability. People see hyperloop, they crave its speed, but the costs are too high and revenues are too low to just serve the endpoints. But if you decide to serve intermediate cities, you slow it down, and it no longer becomes the object of technological lust, and nobody will want anything to do with it.
Among economic historians and really nerdy train enthusiasts like me, this isn't a contentious point. The same economic delusions were a primary causative factor in the overhyping of railways during railway mania, best exemplified in chapters 16-20 of Collective Hallucinations and Inefficient Markets: The British Railway Mania of the 1840s [0], which is a masterpiece of economic history IMO. Anybody in charge of building out some version of hyperloop should do themselves a favor and read it, and then work on not repeating history.
[0] http://www.dtc.umn.edu/~odlyzko/doc/hallucinations.pdf