Even in the 80's & 90's, Sentai was kid friendly. Despite having a serious tone. If Sentai became dark, it would turn off the target audience. Which Go-Busters was condemned for, due to it's poor performance.
Gobusters didn't fail because it was dark, it failed because it got boring after they killed the main villain and erased the point of watching the series by forgetting about their parents and various other plot points. I got bored after episode 10 and then hearing that Messiah died not long after, I decided to ditch the series for good.
Also, the toy line failed in that it wasn't in a singular tone and theme/motif. The mechs were out of place in a series that was going for a different tone, plus the mini robots were mostly useless outside of the base aside from the red one.
With these two things in tandem,no wonder why the series bombed as hard as it did. No central focus, boring characters, a storyline that hardly emphasized spying, and a toyline that was in its own universe divorced from the show. If the series was done like the 80s, toned down the mechs, and focused on intelligence gathering and homaging Mission Impossible, James Bond ,Charlie's Angels and other spy series.
Kakuranger was a better spy series than Gobusters and its not even revolving around that.
The problem is that they can do darker sentai well ala Dairanger, but their execution and the toyline doesn't match up many times, so its bound to flunk. That and Maskman, Jetman, Zyuranger to an extent, Abaranger, a bit of Hurricanger I've watched, Boukenger?, and Magiranger have had serious tones and did well.
The issue with Oh was that the story lost half its braincells since they didn't have a plan B after the bomb attacks in case it wouldn't be embraced as well, Gekiranger had an OK premise, but fell flat, and Gobusters is what I explained above. Ohranger could have been far better had they inverted the cult theme on the villains from that point on or just never attempted it in such a manner.
I feel they should try a family audience and go from there, not necessarily darker or lighter.