[Onelifetogive]

Cost is the ONLY problem.

Storage is another major problem. Supply is intermittent and cannot be adjusted to demand. (i.e. You get power when it is sunny, not when you need it. Luckily, demand is often highest when it is sunny. Business day, people awake, A/C, etc.)

[Uncledave]

Storage is another major problem.

Not so much for grid-connected sites, which is the vast majority of load.