Nadia is, of course, most well-known as Hideaki Anno's prior series before Neon Genesis Evangelion. And the parts of this series that he directed bear some resemblance to themes and ideas that would later be explored more deeply in NGE, especially with regards to religious symbolism. For most of Anno's run on the series, it's a fun, well-paced steampunk adventure series, following Nadia and Jean's travels on the submarine Nautilus, and their fight against an advanced alien race called the Atlanteans (or Neo Atlantis). Some of the world-building is very confused, for example, it seems that the destruction of Atlantis happened just 14 years prior to the start of the series, which takes place in "real-world" 19th century France. However, no one seems to have even noticed that it happened, even though they had supposedly controlled the entire world less than 2 decades ago. There is simply no explanation for how no one remembers Atlantis' worldwide dominance within living memory of most of the population of Earth.
That is a minor nitpick compared to the cardinal sin of this series, which was Gainax deciding that since the show was doing so well on TV, they would just throw almost an entire season's worth of filler episodes straight into the middle of it, bringing the plot to a screeching halt. Anno, realizing how idiotic this was and how badly it would throw off the rest of the series, refused to take part, so it was handed off to a new director that was clearly unfamiliar with the series and its characters, with the animation outsourced to Korea. The result is a nearly-unwatchable stretch of episodes 23-34, nicknamed the "island episodes", where Jean and Nadia are stranded on a deserted island and nothing happens whatsoever. It's boring, but even worse than that, Nadia acts horrendously out of character and becomes intensely unlikable and even brazenly tries to cheat on Jean as a way of generating some cheap drama. If you were to tell me that the new director had some sort of grudge against Nadia and planned the entire arc to thoroughly and gleefully assassinate her character, I would probably believe it. That's how bad it is. And just for good measure, the animation is atrocious for this entire stretch as well.
That ends immediately with episode 35, where Anno is back at the helm, and finally gets the story back on track. You can legitimately skip straight from 22 to 35 without missing a beat, though there is a little bit of context in 31 that may be helpful for the ending. Anno in fact seems to have designed 35 almost as a repudiation of the island episodes just by making the transition from the pre to post filler arc so seamless that they may as well not have even happened. The ending to the story is solid, though some parts were rushed and I would have preferred that (SPOILER ALERT) the reveal that the Atlantean-supremacist Gargoyle was actually human would have happened earlier in the story, with his self-loathing serving as context for his behavior, rather than it being thrown in off-hand, almost as one final insult while he's already dying.
In the end, the story it tells is pretty decent if you skip the filler episodes, but this is the series that will always have the legacy of making Hideaki Anno so depressed and disenchanted with the industry that it heavily influenced the dark tone of NGE, which was originally planned as a sequel to Nadia before having to be rewritten due to Gainax's loss of the Nadia IP, several years later. Nadia probably deserves a higher rating on the merits, but you can't simply ignore the island episodes when fairly appraising the series as a whole (though I highly encourage you to ignore them as a viewer).
