Everything the show did well:
-The team members have likeable personalities overall.
-The main villains being a monarchy where the generals are in-fighting for control is unique and pretty entertaining.
-Maskman fighting styles are really unique with their combination of emphasizing martial arts, more varied weapons and their occasional use of power-construct abilities.
-Whenever the show has budget, the monsters of the week have the gimmick of being divided in two, the body and the mask. This creates an interesting and very different dynamic to fights both inside and outside of mechas.
-Great acting overall.
-Cool soundtrack.
-The stakes feel more personal for the red ranger Takeru due to revolving around saving his girlfriend.
-Cool world-building for the most part.
-The Light Squadron is really cool and manage to be really useful for the most part.

Everything the show did bad:
-The show is too red-centric. The majority of the first quarter focuses too much on the red ranger Takeru. Most of the villains rivalries also revolve around him. What's worst is that despite of this we don't get a proper look at his backstory until really late in the show. We don't even know what's the pink ranger Momoko's deal until episode 15. As good as the rest of the team are, they just are way too heavily shafted throughout the whole show.
-The show has too many villains simultaneously which makes it really hard to care for them. It also feels like the show doesn't get to properly use its ideas for the villains because of it.
-It felt like the Unas arc could have been longer.
-It felt like commander Sugata's backstory could have been explored more.
-The plot twist behind the Main Antagonist is pretty dumb and barely changes the status quo.
-When the show doesn't have enough budget (which is the majority of the show) the monsters of the week tend to be really generic and forgettable nor do they use the mask gimmick which feels out of place since the show doesn't explain why. And even when the gimmickless monsters are actually interesting, their mecha fights sucks since they barely retain anything of what made them interesting once the become giant.
-The main antagonist Zeba, Anagmas and Okelampa are really boring villains.
-The show's finale feels needlessly depressing and honestly kinda sucks.

Reasons for the show's strengths:
The staff wanting to slightly move away from science fiction definetly gave the show more originality and allowed to try new things.

Reasons for the show's problems:
This was the 6th show that main writer Hirohisa Soda had done in a row without breaks. By this point there was starting to be a lot of overlap, specially considering the nature of these type of shows and how Toei during this period was more comfortable to listening to sponsors. The next show Liveman was able to try more original things due to being the franchise anniversary but after that the franchise would go back to being too samey. But even when this show tried doing original stuff, it tried to do too much at the same time making it so that it couldn't properly develop their good ideas and leaving them feeling half-baked.

Overall:
It's not a bad show overall and definitely has ton of interesting concepts. But it's just so heavily flawed to be considered a great a show. I feel that even if you really like the concepts it will only bring it up to a 7. It's a pretty good show but I only really recommend it if you're really into Sentai or if the setting sounds really interesting to you.