They weren't completely useless, but mechanics needed to know what they were doing to get them working adequately.
Many big professional races were won using them around 1987-90. Laurent Fignon rode with them extensively for about three years, winning Milan San-Remo and the Giro d'Italia, and out-riding Greg Lemond for most of the 1989 Tour de France, with them.
Stephen Roche won his Giro-Tour-Worlds triple crown using them for most of the season, including mountain stages where serious braking capability is required in racing scenarios. Interestingly, though, on the stage to Morzine, where he blitzkrieged away from Delgado on the tortuous descent of the Col de Joux-Plane into Morzine, he had switched to C-Record side-pulls. Delgado went on to win the 1988 Tour using Deltas, but was back on side[pulls in 1989.
Some interesting reading
HERE
In any case, it was a daft idea to produce a brake that was so awkward and labour-intensive to adjust and maintain, when simpler superior, lighter options already existed.
Of course, by the standards of today's superb dual-pivot brakes, especially when with the very low-friction cables on the Ultegra/Dura-Ace level of components, all of the rim brakes of that era were on a lower level. Shimano began the real drive for better rim braking when it came out with its excellent Dura-Ace 7400 series brakes (single pivots first, then dual-pivots). And the difference modern top-level cables make, compared to back then, should not be underestimated.