As this question predated the Sea Mauville episode, I'll make some notes about which element came before and after that, though for the most my feelings didn't change much.

I feel Project Mew was one of the better ideas JN had, but also one that showed the series's flaws very clearly. To go in the specifics, I believe the concept itself was a great call: while Goh had several good episodes by then and was a likable character with realistic flaws, the way his goal was handled wasn't the best: not so much for the method and amount of catching, and more for the fact simply catching Pokémon to metaphorically get closer to Mew didn't allow his character a lot of room for growth beyond episodic adventures. Project Mew allowed to reframe said goal in a way more resembling prior parallel quests, making it a better match for Ash's side of things: rather than having them both making progress in incidental parts of another's episode, this allowed for a clearer set up of 'Goh goal progress episodes' and 'Ash goal progress episodes', which provided better structure. Paired up with some downplaying of Goh's catching (specifically, while he does keep catching Pokémon, after Absol he never has entire episodes built around catching eventual minor additions to his team), this helped giving Goh the direction he needed while downplaying the weakest elements of his character and goal.

However, this arc also puts into stark display how poorly planned Journeys was as a full series, as not only Project Mew was never part of Goh's goal narration the same way Ash's PWC climb is, the most important episode to set it up (Goh's capture of Suicune) only happens fifteen episodes before, with Horace taking quite a bit longer to tie back into it afterwards.

This feeling of a hastily put together plotline is unfortunately keenly felt throughout, as characters like Danika and Quillon have only hints of backstory at best in spite of owning legendary Pokémon, and Professor Amaranth gets very little focus for the man in charge of the whole expedition. This is even felt in the first few episodes, as Goh's earliest missions made sure to include Ash even at points where it would've probably been best for the story if he weren't involved (the offscreen Regice battle and having him included as the only non-challenger in the Articuno raid battle being notable examples), which was only accounted towards the end around the time the show also introduced more rivals. It felt like the show was still figuring out how it all would work well after it started, and the cracks at the seams are hard to miss.

But in spite of that, I do think Project Mew ended up telling a pretty fulfilling story, bringing focus to Goh's character flaws and issues working with others, alongside providing interesting scenarios. Introducing Gary to act as his rival was an inspired choice that keeps up Gary's shift to researcher from prior series to the forefront while not feeling like a regression to his character, and I particularly liked how Gary and Horace were involved. While this happened after the question was posed, I feel by the final episode the structure of Project Mew was crystalized enough to provide interesting challenges (and while they had small roles, I did like Sterling and Lyla as minor more antagonistic rivals), and while Regigigas ending up as just a cameo rather than a major challenge was a bit weird, I feel the exploration of Faraway Island proved a nice enough capoff to Goh's arc.

So all in all, I feel Project Mew was a flawed but much needed and ultimately fulfilling addition to JN, and one of the reasons why I consider Goh's character and arc a highlight of the series.