Hunter X Hunter's were like that because the anime caught up to the manga. If you compare the old anime to the manga or new anime, there're a lot of addition scenes or scenes that are stretched out. HXH handled it really well by having filler that adds to the series instead of delaying story progression. Just look at Dragon Ball Z taking 10 episodes to show Goku charge up or filler arcs from Kenshin, Naruto, or Bleach. Eventually it hit that point where they had to end the series. The anime ends in the middle of the yorkshin arc and the first OVA, which is excellent, finishes it up. The final two dealing with Green Island, is when they rushed the animation and everything dropped in quality.
Kenshin's case is a weird since the first set is essentially a backstory to the final arc, and it was all done in regular Kenshin/children comic style, so overall lighter. But since that final arc wasn't animated, that backstory was adapted into the darker, more adult oav. The final arc from the anime was all filler. The second oav was a really loose adaptation of the final manga arc done in the style of the first oav and it sucked. And the first oav is really great. So that whole battosai arc probably ended up better than it would have if the anime hadn't caught up the manga.
these are hunter x hunter spoilers, but just to compare the series to the latter oavs:
spoiler (click to show/hide)
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