True supersonic deltas have a VERY low aspect ratio - Extremely chord-y, but very short span. This causes them to be quite inefficient in high alpha flight. Control inputs, especially elevator, are less efficient due to the single lifting surface having to provide for competing goals of lifting the nose and depressing the tail - Elevators effectively modify the pitching moment of the wing's airfoil rather than providing a simple down moment on a conventional tail, while the wing lifts, as ever. Flaps are impossible on a true delta, so approach visibility is very poor - The pilot is looking to the sky, the nose blocking the runway. High energy turns throw away ALOT of energy in induced drag.
The Mig 21 and F4 have semi-delta wings, and are significantly less chordy than most true deltas. This lessens the trade-offs the delta imposes, while maintaining much of its advantage in supersonic flight. The more conventional tail allows more conventional maneuvering loads, and allows for various flaps to be applied to the wing, reducing required alpha at low speed segments of flight. Much less energy is thrown away in a turning engagement.
Every airplane is a compromise, and wing planform holds a host of them.
J