To use non-technical expressions for all the aeromodelers on here claiming to be pilots, the Hawk is smooth, lovely and swoopy. The '21 is rough, pointy and bumpy. As for the F5, it's an interesting design, beautifully finished and very neat internally. But in the air it lacks something. I think it's called 'balls'.
Like all highly capable aeroplanes, the '21 is very difficult to fly excellently, hence its shocking record in the hands of our cousins in the Middle East. Sadly, they couldn't even use the systems properly, far less fly the thing. The places where a '21 really excels are on the outer edges of its flight envelope and that's where it will live in the hands of a half decent pilot.
No-one thinks a thin aluminium skin going to protect them from nasty flying objects. It's a psychological issue; about where you feel you belong; about having the aeroplane wrapped around you While I think (though can't confirm) that the view from the cockpit of an F15, 16, et al., must be just glorious as you swan around the sunny skies of Nevada, it's not where I'd want to put myself when things get iffy.
Having enjoyed a ride in an F4 I can honestly say I like the giant lump. It seems like a honest machine. I'm certainly not qualified to offer an opinion on its systems, but they too appeared 'competent'. However, as so many others have pointed out, it's not a fighter in the strictest sense of the word and it never was a true competitor for the Mig 21. But an early '80's '21 vs an early '80's Lightning? Golly, that would have been a giggle....
Anyway, we're all old now and arguing pointlessly over things past. As we're a few days into - for us - the Year of the Cat, I'd like to wish you all 'Chuc mung nam moi!' Happy New Year!