It's not really about power level, its about having an interesting ability with interesting counterplay.
To be honest, it is. Signal relay is more akin to tracking, armored detection, infantry detection (or whatever the OH SC vet ability is called), radio silence, radio intercept etc. All which interact directly with minimap information but doesn't provide LoS to benefit other units from it. This and only this aspect is what makes or breaks the ability.
It's all about powerlevel. No one complains about Overwatch or radio intercept, because even if they are passive free abilities, the usefulness is limited.
Signal relay is a useful ability, on a bad commander. Same case with Spy network (Partisan).
Artillery flares are the best recon tool (uncounterable and doesn't provide the same feedback as a recon plane so you are less prepared for it, if used in the backline) in the game, in one of the most meta commanders across the OKW life. If it was say part of Overwatch or any commander focused on intel with a niche more use or not as good, it would probably be overlooked or argue that you have to pick a "weaker" commander in order to enable other allies offmap with vision.
Nerfing the ability depends on what you want to accomplish.
-If the main component is intel and not boosting/enabling other offmap you can change it to be cheaper and provide minimap info or tracking of the units in a zone without providing LOS for offamps.
-If it's to avoid constant bombarding of onfgoing abilities such as loiter planes and ramping artillery, the LoS time should be reduce drastically while still giving opportunity for other type of "1 shot" offmaps.
-If you want to keep it as it is and the ability is consider OP, increase the cost of it to be highest of all recon tools.