I am no engineer, but Brahmos 2/zircon going Mach 7 at sea level is probably producing a lot of heat on the leading edges. Aside from the exhaust which would also be considerable,, this is a big non stealthy beacon for the intended target to see.
It is also very hard on control surfaces, perhaps requiring ceramics, and certainly restricts maneuvering in the terminal phase at those speeds. Also, guidance would have to be without optics, I would think (again because of the heat), and even radar guidance would require heat shielding and complicate operation.
The Mach 7 sea skimming missile reminds me of the Shkvall torpedo, fast but unguided (or poorly guided), more impressive to the uninitiated than useful, but again, I am just an armchair engineer.