Aerodynamic question

Back around 1960 Manuf seem to have problems with directional control at higher mach numbers. I have read for instance that the F8U would begin to get unstable around mach 2. That is why the two strakes were put under the lower aft part of the tail. Many F-100s crashed because of this untill a larger fin was put on them. Vought even went so far as to put the two moveable tail planes that went down when the gear came up. Basically it made the aircraft have 3 rudders.

Modern planes do not apparently have this problem. So my question is what engineering fixes do modern planes have that make them directionally stable that a 1960s planes didnt have? I know of course that many have dual verticle stabilizers. Is there anything else?