I agree with much of what you say.
"The Hs 129 has a bad reputation as people believe that its engines were unreliable and underpowered. Neither of the two is true. It is one of those horrible stories... The Hs 129 was the best armoured aircraft of WW2, it had reliable engines."
I think part of this story is from it's use in North Africa. ANYBODY'S aircraft in NA were unreliable and short lived without good air filters. Lack of spare engines and parts ( no other aircraft in the theater used the engine?) meant that serviceability would be low, contributing to the story?
However power was, shall we say, marginal? 1400hp for a 10,500-11,500lb twin engine plane is not particularly good. It is better than than the Blenheim V but not as good as the Blenheim IV. Adding high drag large gun pods underneath didn't help. the 30mm pod might not be too bad but the "famous" or infamous Bordkanone BK 7,5 cannon ( it's reputation far exceeds it's actual use) was pushing things over the top.
Worn engines producing less than book power ( and I don't doubt there was some sabotage) can contribute to the story.
Many aircraft acquired reputations based of a relatively few incidences, for good or bad.
For a low altitude gun armed "strike" aircraft a number of things come into play that are not shown by a short list of performance numbers. How fast an aircraft responds to the controls is VERY important, both for aiming and avoiding unintentional contact with the ground. Good throttle response is helpful. Not in the sense of absolute performance numbers but in the sense that when the pilot does something the plane responds right NOW! with no hesitation or delay.