I think Sun and Moon was really good. The cast was large and allowed for a lot of different combinations and dynamics. Not all of them were that deep, but it hit a lot of good notes. The bonds between Mallow and Lillie or between Mallow and Lana are very well done. We also get some great character stuff from stuff like Sophocles' perceived goodbye episode or the episode where Rotom might leave (where Lillie supports Ash).
Journeys sometimes hits good stuff, but sporadically and usually with outside characters. The dynamic between Go and Ash just isn't really working well as they don't complement each other that well. Sure Sun and Moon also didn't have deep interactions between all the characters, but when you only have two main characters at the forefront you can't get away with variety as much. Go offers nothing to Ash. Ash sometimes can help when Go has issues with his pokémon, but even then he doesn't do much. There's never really any depth between them, when an episode is about something Go is going through Ash is just there in the background and when an episode is about Ash Go is just there in the background doing nothing, or maybe just catching a pokémon that'll never be used. The few episodes I've seen Chloe actually appear she seems to have the potential to provide that depth, but the story just doesn't do anything with her and she's hardly ever around, as a result her interactions with Ash and Go are mostly limited to her just showing her disinterest in their activities.
It's not so much about the amount of characters, it's about how they interact and are utilised, and whether their characters can complement one-another through their different perspectives and provide depth. Go and Ash have different goals but their characters just don't bounce off each other all that well.
Pokemon chemistries are also of note in Sun and Moon.
In SM nearly every Pokemon team had a good number of fun chemistries, which meant they could keep even balance between each other.
Ash's team for example largely feel balanced and all get reasonable development in the league, largely because all of them have dynamics with each other or rival Pokemon to make themselves relevant. Compare this to say XY, that started off good in this area, but slowly just became the Greninja Show after the Pokemon evolved and no longer had a synergy. SM was also good at linking development between characters, eg. at least four Pokemon wanting to evolve to keep up with one of their teammates, or being caught because they befriended a Pokemon first, Ash and Kiawe helping Sophocles train Vikavolt, Ash and Gladion helping Lillie with her phobia. This is more than just a pep talk or cheering from the background, these characters are getting involved directly with another's growth.
This is generally why a main character has to have a wide range of chemistries with the rest of the cast to give them openings to develop and gain a role, even when it isn't necessarily their turn in the limelight. There was a larger amount of 'duo' episodes in that series more than others because of this as well. Ash/Lillie, Ash/Kiawe, Kiawe/Sophocles, Mallow/Lana, etc. and often times these were some of their best roles.
I think this is why people put Dawn so high on the list for example, she not only got a good individual arc but she was generally good at gaining chemistries with other characters to involve herself as a supporting character. There's not quite as many 'background prop' roles for Dawn yet she allowed other characters to shine (besides arguably Brock).