To be fair the whole dark/light side is stronger/weaker is a bit sketchy. In canon I can only remember Yoda pointing out that the dark side is not stronger. And then pretty much every Sith saying that it is actually stronger. People tend to accept Yoda's version like absolute truth, but we don't see much evidence to support it.
First Yoda himself might believe so, but that doesn't make it true. How could he know really? He doesn't use the dark side. A bunch of important fights in the movies are won because said person uses the dark side (Luke vs Vader or Anakin vs Dooku). Yoda gets his ass handed to him by Sidious, and he has more than 800 years of experience in using the 'superior' light side. Where is this 'light side stronger in the long run' mantra here? How much more long run than this can you possibly have? Yoda when he confronts Dooku says that Dooku has become stronger and that he senses the dark side in him. Seems to imply that the dark side made him stronger. We see plenty evidence of dark side making people stronger fast but where is the evidence of making people weaker in the long run?
Yoda also could be lying to Luke, just like they did with the Vader situation, so he would be more confident and not be afraid of fighting Vader/Emperor.
How about the fact that in canon they pretty much always end up losing in the end?