The fact people can’t use their cannon should not compensate.
The F3H Demon is balanced by the fact it needs to either be shoved off a cliff or at high altitude to get supersonic, and the missiles it does get are AIM-9Bs and the very early AIM-7Cs which are practically useless at anything other than high altitude.
The F11 Tiger gets balanced by the fact it has a horrendous turn rate because of the swept wings, and the fact that the engine is, relatively speaking, not that powerful. Afterburner is pretty much mandatory.
The F-104 is not “better” in every way. The F-104 has a good gun, and is comically fast, but the price it pays for it is that it is horrendous at turning in every sense of the word above 750kph.
The MiG-21, especially the early versions, are “challenging” to fly as they too can lose speed exceptionally quickly, but they compensate for that by being able to turn very hard when it has speed, and getting a good hard hitting weapon systems and 2 R-3S missiles.
Basically, skill issue, learn how to handle your plane. It’s crime enough that they’re 9.3 (F-104 and MiG-19 included)
The Sea Vixen cannot engage from the front aspect. The MiG-21 can.
Because players do not understand the flying characteristics of the aircraft they fly. The MiG-21 F-13 is exceptionally dangerous if handled well and not facing all aspect missiles (shocker). It is limited by 60 rounds, but good trigger discipline and understanding of the ballistics of the gun will aid with it. Same problem with the PFM (I’ll admit to struggling with the GSh-23 gun, since that thing needs a lot more lead than you’d expect, coming from an ADEN user). Throw in that it is very easy to lose all your speed very quickly, which makes you a free kill in a furball, and it is a very difficult plane to fly. Doesn’t mean it should be undertiered.
This is universal to other 9.3s and 9.7s. See the Jaguar GR.1, which while it does have 9Gs, will commonly face aircraft with flares, and has even worse performance than a MiG-21. Or the French Hunter which iirc is 9.3 with 9Bs, not great turning, and firmly subsonic.
It’s a bit like the F3H you mention later - they turn very hard and consequently run out of speed and energy quickly. Speed is your friend in the MiG-21. If you have to turn, avoid making it sharp.