Note: Images from Super Sentai Images
One of the complaints that some Sentai fans have about Goseiger is the multiple villain groups. Personally, I don't have a problem with it nor with Bladeron staying throughout the season. It does keep the story fresh and moving. It's been done before in Sentai, actually. The problem is how they handled the use of the multiple villain groups. Here, I'll analyze how three different Sentai series, Magiranger, Boukenger, and Goseiger, handled having multiple villain groups.
Magiranger was arguably in this category, with three groups or commanders of Infershia: Branken, Meemy, and the Hades Gods of Infershia. Similarly to Goseiger, they all came one after another. After one was killed off, another would step to the plate. However, unlike Goseiger, all these villain groups belonged to the same organization, Infershia, under the control of N Ma. They all worked to revive N Ma to conquer the world. Also, Wolzard and Vancuria/Nae & Mae were constant throughout the season. Since the three different villainous groups/commanders in Magiranger belonged to the same organization and had an overarching villain, transitions from one villain or villain group was smooth and there was no early climax. Also, Wolzard imo is one of Sentai's best generals, and had an interesting rivalry with Kai. Among the three villain groups I thought Branken was ok. He wasn't a good villain or a bad villain, but descent enough to start out. Meemy I felt was the second most interesting villain in the series behind Wolzard. Meemy was a former Heavenly Saint who was also a traitor. I liked most of the Hades Gods, especially Sphinx and Titan. The only one I didn't really like all that much was Toad. So overall, all the villain groups in Magiranger were pretty solid. I think Magiranger handled the multiple villain groups really well.
Boukenger like Goseiger and unlike Magiranger had multiple villain groups that were not connected through the same organization nor had an overarching villain. However, unlike both Magiranger and Goseiger, these four villain groups didn't come one and go after another was defeated. Instead, they were all constant throughout the entire season. The Boukengers would fight them in separate episodes and sometimes these villain groups would team-up. None of them were killed early in the season and thus there weren't any early climaxes. The villain groups were the Gordom Civilization lead by Gaja, the Jaryuu Tribe led by Ryuoun, the Ashu/Questers, and Dark Shadow. What made it interesting is they all had different motives. Gaja and Ryuoun each wanted to conquer the world, the Questers were human hating demons, and Dark Shadow was a ninja clan that was more interested in making money off of the precious' they stoled. Among them, Gaja was probably my least favorite. He wasn't a bad villain he just felt kinda bland and stiff, and I felt it was anti-climatic in the final battle when he went back to sleep and seemingly gave up. Ryuoun was awesome and had an interesting backstory that really developed his character. The Questers were even more badass than Ryuoun and had a history with Eiji. Dark Shadow was refreshing because they weren't bent on conquering humanity like the other groups, they were money-driven instead. Yaiba was the most interesting of the group, eventually betraying Dark Shadow. Shizuka was the one in Dark Shadow that didn't have much backstory or development, but I kinda liked her somewhat childish behavior. What really made having multiple villain groups at once interesting was that all the Boukengers except for Natsuki had a villainous rival. Satoru's rival was Ryuoun. Eiji's rivals were the Questers, especially Gai who killed his father. Masumi's rival was Yaiba. Sakura and Souta were both rivals with Shizuka. Like Magiranger I felt Boukenger handled the multiple villain groups really well and arguably better than Magiranger.
Then we have Goseiger. Like Magiranger, Goseiger's three villain groups (Warstar, Yuumajuu Beast, and Matrintis Empire) all came one after the other was defeated. Unlike Magiranger, however, there was no overarching villain or organization connecting the three groups. However, there was one constant villain, Bladeron. Bladeron was the most interesting villain on Goseiger who like Meemy, was a traitor and a rogue Gosei Angel. One of the big problems with how the multiple villain groups were handled in Goseiger was early climaxes, with the most obvious example being Warstar. Through the first 11 episodes, Warstar does nothing and felt rather bland and boring. There were hints of a rivalry between Alata and Dereputa, but Dereputa was killed off initially in episode 12 and for good in episode 16. As a result the rivalry went nowhere and Dereputa came off as wasted potential. Similarly, Warstar's Big Bad Mons Drake also came off as wasted potential. After Dereputa was initially thought to be killed off in episode 12, Mons Drake got desperate and made the show interesting briefly. However, Mons Drake was killed off in episode 15, which was another early climax and a big disappointment. This also didn't help with audiences since there weren't hints of Goseiger having multiple villain groups after Mons Drake. However, the worst part was to come, with the Yuumajuu Beasts. The Yuumajuu Beasts are easily one of the worst villain groups in Sentai. The Yuumajuu Beasts were pollution-driven villains, which I really despise. I'm a bit biased here but having pollution driven villains imo really feels so anti-climatic, uninteresting, and non-threatening. This was made worst with a really preachy 6th ranger, Gosei Knight. Some fans complain that the Green Aesop in Go-Onger was overt and preachy, but it was nothing compared to the combination of the Yuumajuu Beast and Gosei Knight. Not to mention the designs I felt were terrible. The Yuumajuu Beasts were killed off after episode 32, but this time it didn't feel so rushed or like a premature climax. However, that's because you could naturally expect it to happen after Warstar being killed off so early in the season. Then finally the last villain group was the Matrintis Empire, a robotic empire that first appeared in Episode 33 after the change of producers. I actually liked the Matrintis Empire. Among the three Goseiger villain groups they were the most interesting and had the most effort put into them. They went back to the old conquering humanity motive, but I felt like they were the only legitimate villains in Goseiger besides Bladeron that felt like a threat. Unlike the other Goseiger villains (and many Sentai villains for that matter), they scouted the Goseigers, strategized, and collected data. They actually did their homework. Robogogu had a mildly interesting backstory of being a former mad scientist, while Metal-Alice felt like a throwback to Sphinx of Magiranger. However, she didn't feel so much like a throwback that she came off as a rip-off. She seemed interested in the idea of having comrades, but mostly because she was abused and unappreciated by Robogogu. And unlike Sphinx she didn't turn good in the end, but was killed off by Bladeron.
Having multiple villain groups without an overarching big bad could have worked in Goseiger if it was better handled. Overall, Goseiger's handling of the multiple villain groups was easily the worst. Transitions often felt rushed, and the Yuumajuu arc was really the jump the shark moment of Goseiger, even more disappointing because it covered the entire middle portion of the series and after it briefly became interesting for a few episodes. However, I think there's also a clear difference in the Matrintis Empire, who came right after the change of producers. I felt that wasn't a coincidence either.