^A lot of /m/en including myself find the mecha top tier and the villains are unique by sentai standards as there are four of them with different goals and a few of them are alive by the end compared to most sentai. Gokaiger could have used that giant owl demon again, but no they had to resurrect a dead one for some raisin. I know Dr. Kain won't respond to this because I prefer evidence to mere hearsay, I will say he is at least trying and I won't fault him for that.
I never really found Boukenger mech too appealing even back when I liked show more. They all look way too busy and show signs of overdesisgned clusterfucks that will plague franchise during 10s. Though DaiVoyager does look pretty good, I guess.
As for the villains, I like the idea of different groups acting simultaneously, but none of them had enough presence or felt like a big enough threat to heroes. Huge mistake was to downplay their tensions with each other as much as show did. They all feel more like different brunches of the same villain organization (which they are for Boukenger, but shouldn’t be for each other), not four distinct self-sustained ones which always supposed to compete not only with Boukenger but also with each other. Infershia Gods from just the previous year had more interesting relationships and they were a single group.
They also lack their personal stories that could have made them more memorable. I liked Questors and Silver shared backstory initially (it almost seemed like they came from their own show, which when I first watched I thought was pretty cool), but show dropped all Ashu elements of them both pretty much right away until it was the time for final confrontation with Questers. Dark Shadow had only something going for them in the end with Yaiba betrayal and Gekko backstory (I’m not counting Yaiba’s dumb tauntings of Black’s “darkness” once in 10 episodes as a “story”). Both they and Ryuuwon were mostly butt of the jokes for the majority of the series. And Gaja was just letting everyone to take advantage of him, left and right. These guys just aren’t menacing at all and they aren’t very interesting either.
Boukenger could have taken a page out of Might Gaine’s book, which had similar set-up of four different villain groups. I liked how initially each villain brought his or her own flavor to episodes. With Wolfgang you had classic old-school mech motw (and later rivalry with Joe the Ace), with Hoi Kow Lou you had small conspiracy plots, with Mifune you had big flashy fights that pushed Might Gaine to his limits and with Catherine you had wacky comedic plots. Going up against each villain felt very unique and not just because their methods were different. Then Might Gaine gradually moved them aside to make way for “the big villains” of the show, while also giving opportunity to flesh out most of initial bad guys more and even give some of them development to the point where Wolfgang ended up being my favorite character in the show. This just makes Boukenger villains look mundane and boring in comparison.
As for Gokaiger/Gekko, they did bring him and Shizuka back in Gavan crossover, but they were somewhat small-scaled villains even by Boukenger standards, not to mention show just randomly decided to make them more sympathetic than others in the end for some reason. Gokaiger probably wanted someone more menacing for Boukenger tribute, so they went for Ryuuwon (I’d have preferred Questors, but I guess they wanted someone with more personal connection to Akashi).