The thing is, especially early on in the V6T Hybrid era the Mercedes engine was miles ahead of everyone else. There was simply no other engine capable of winning championships.
The first few years Mercedes was running the engines downtuned to lose some performance, because they felt the gap so huge that it would make the sport look bad if they were running a second a lap faster than the second team on the grid. With such a gap the FIA would also surely intervene fast. Artificially keeping the gap smaller, and only turning up the engine when needed gave Mercedes easy wins, while keeping the FIA away from intervention.
The fights became closer when the other manufacturers started getting more performance out of their engines.
Last year Red Bull won because the FIA changed some aerodynamic rules. Everyone expected that the changed rules would favor Mercedes and Aston Martin who were running low rake. It turns out the expectations were false, the new rules impacted Mercedes and Aston Martin hugely and the teams running a high-rake setup didn't lose all that much performance. This gave Red Bull a edge which lasted until around Silverstone, when Mercedes brought upgrades to fix the performance of their car.
The championship lead built by Red Bull was just enough to win the title in the end with some controversy in the final two races. There never were rumors of cheating anywhere.