"Unbeatable"?
In the case of the championships, maybe. But the FW14B and 15C, for all their technical wondrousness, weren't the most reliable cars F1 has ever seen though. And they were beaten to the win on a number of occasions - the aforementioned Schumacher getting them once in '92 and once in '93 (at Spa and Estoril respectively), Senna getting three wins in '92 and five in '93, Berger nabbing a couple in '92.
So six wins in 1992 were made in cars that weren't a Williams. And six again the next year. 12 out of 32. 37.5%.
I do agree that they werent the most reliable, compared to the cars being produced today, where you have a McLaren not breaking down for an entire season. However, during that era (1992/1993), the Williams were as reliable as any other F1 car produced.
Secondly, when a car other than a Williams was winning in 1992/3, it was due to either a Williams breaking down OR sheer brilliance from the race winning driver.
Lets go into detail:
1992:
Mansell wins first 5 races
Race6: Monaco, Mansell comes in for an extra pit stop, as a wheel nut wasnt tightened. When he comes out he is stuck behind a much much much slower Senna. In those days, the cars were wider, so there was nothing Mansell could do.
Race7: Mansell crashed and Patrese had technical problem. Hence, Senna won. Take note, this was the only race that season that a Williams was not on pole. 15/16 poles is a very very high success rate.
Race8,9,10: Mansell wins all 3 races, including Silverstone, during which he set his pole position lap, on a dry track, which was about 2.5s faster than the next non-Williams car and 1.9s faster than his team mate - a feat that we have since never seen.
Race11 (Hungary) - Patrese in a Williams wins.
Race 12 - Mansell leads for much of the race, but then it starts raining and that man, Michael Schumacher wins his first race.
Race13 (Italy) - Patrese led until the close to the end of the race, when his car broke down. Mansell also broke down. This allowed a non-Williams to win.
Race14 - Mansell wins.
Race15 - Patrese (Williams) wins.
Race16 - While leading, Mansell is shunted from behind by Senna. Both retire.
Williams secured 15 out of 16 poles.
Williams scored 164, where the 2nd placed team score 99.
Mansell secured his title in only the 11th race of the season.
In an unprecedented move, arguably the best driver of the time - Senna - announced he wished to drive for Williams, for free, as the Williams really was head and shoulders above any other car. By far.
I could do the same thing for 1993, but you get the picture. When Williams turned up to a GP meet, they were expected to get pole and win the race. The other competitors were hoping for a Williams break down. Thats all they could hope for.
The highlight for me that year was Mansell's qualifying lap at Silverstone. That 1 lap told us just how fast the Williams was and how hopeless other teams/drivers must have felt.