Well I can tell I may have tripped over something here so I'm going to venture forth cautiously.
I tend to look at these things from an Operational point of view. I don't know the full operational histories of all of the Century series fighters but all of them seem to have fallen into one of two categories when seen from that angle:
1) Interceptors or fighters with limited utility that were never ready for prime time and got relegated to home defense. Generally disliked or mistrusted by pilots and plagued by low servicability.
2) Fighter bombers which had some operational utility but suffered high loss rates and other significant limitations.
In both cases there seemed to have been extended and serious development problems with fatal accidents, as well as some infamous corruption scandals.
I admit the MiG 21 was a fairly ugly plane (and I think this is true for the -17, and -19) with short range, and no doubt it too had teething and reliability issues, but it seems to have had a good combat record and a very long combat service history during which it remained viable.
When you are looking at things from an operational point of view it helps to consider a number of operational facts. It also helps not to go in with preconceived notions (US aircraft were expensive crap foisted off on the US taxpayers).
In the 1950s they were just starting to get into life cycle accounting (how much it costs to operate a plane or fleet of planes over a number years or hours of operations). The Russians didn't tell anybody how few hours per year they were flying a lot of their aircraft. If you only fly a few hours a month even short overhaul life engines can last quite a while.
For true comparisons you need to know how many hours were being flown per month (or per year), how often engines had to replaced and other
operational factors.
US readiness rates sometimes sucked, but lets compare apples to apples, the f-102 and the F-106 were all-weather fighters from the start, they were expected to take-off and intercept incoming bombers at night in rain or snow storms,. Granted with was with aid (a lot of aid) from ground radars and ground controllers but it took a lot of electronics and the electronics of the day was vacuum tubes and circuit boards. Several cubic feet of 1950s electronics will now fit on one micro chip.
The first all weather "interceptor" versions of the Mig-21 didn't see service until 1961. almost two years after the F-106 started to enter squadron service. As electronics got smaller and lighter you could fit more capability into existing aircraft.
The Mig-21 day fighter didn't enter production until 1959 (?) so it was a tad late compared to the US century series (5 years later than the F-100) one would hope it was better than the older American aircraft.
The F-102 and F-106 got
relegated to home defense because that was what they were designed for, to work as an integrated part of the home defence system to stop Russian nuclear armed bombers from making it to US soil. Dog fighting cheap daylight only enemy fighters was NOT part of the mission requirement. The F-102/106 needed a in plane radars, fire control computers that could compute collision course intercepts and firing solutions for the guided missiles of the day. and a communications link that would allow the planes navigation and intercept systems to communicate with ground systems. The powerful ground radars and computers would vector the F-102/106 to intercept points.
It has been said that the F-106 could be flown from the ground with the pilot only needed for take-off and landing and perhaps to confirm the target identity before firing the missiles.
And do it using those late 1950s electronics (when they worked)
A lot of the century series had much more ambitious goals than the Mig-21. granted they did not always live up to expectations but in the 1950s the Mig 21 either didn't exist (as a production aircraft) or existed in a rather primitive form. "Avionics included PUS-36D weapons sequencing module, R-800 communications radio, ASP-5NV-U1 computing gunsight, and SRD-5MN
Baza-6 radar rangefinder. "
The F-86 used a radar rangefinder from the start in Korea, 7-9 years earlier.
Now I will note that the short overhaul life of the engines used in the early Mig 21s was a deliberate choice. The USSR did not have the access to or expertise to work with some of the exotic materials the Americans and British did. To get the combination of power to weight that they wanted with the material they had to work with engine life had to be sacrificed.
THey could have made longer lasting engines but only by making them heavier for the same power.