The treatment of mythicals in Gen 4 was really really bad.
When people talk about these guys being bland and mediocre nowadays Gen 4 is commonly cited as an example of a "golden age" for mythicals, when all of them had really good events and just better treatment across the board. And don't get me wrong, modern mythicals do suck and often feel totally disconnected from the region in question unlike these guys. But honestly? Basically every mythical in Gen 4 had their own unique, unprecedented problems that put huge dampers on what they tried to be. The entire concept really is just fundamentally screwed, but let's set that aside and talk specifically about where Sinnoh fails.
The issue with Manaphy is two-fold. The first is its obtainment method: Making it a bonus for Pokemon Ranger? Cool! Making it literally one per cartridge? Gross, you had a chance to amend the artificial scarcity horseshit and ya bungled it up. The second, far bigger problem is Phione, which I won't go into in major depth cuz its a dead horse at this point. I just... Don't know what the end state of Phione's conception was. Even
if it could evolve into Manaphy I'm pretty sure it doesn't even get any unique moves that its big brother can't learn on its own. You can't even call it Pokedex filler since it wasn't in either the DP or Platinum regional Pokedexes and it doesn't bring the one-current National Pokedex count of 493 to any really notable milestone (if hypothetically Gen 4 added enough Pokemon to get to 499 and then Phione was created solely to cross over to 500 it would still be dumb but there'd be a reason behind its existence).
Why. The. Fuck. Was. This. Split. From. Cresselia?! They are literally no more or less connected than any other minor legendary duo, so why did they do this? The only other two mythicals connected to normal legendaries are Mew and Keldeo, and the former is deliberately mysterious and ethereal while the latter at least has a "special" role amidst the Swords of Justice for being their student with a special form and signature move. Darkrai is just Cresselia's counterpart. That's it. He ended up being the much more popular of the two, sure, but they couldn't have known that during DP's development and even if they had a crystal ball that still doesn't justify the weird ""favoritism"".
Now, there's nothing particularly offensive about how Shaymin was handled so that won't get a section. But then there's the absolute motherload...
I already knew this long prior, but not until the advent of Pokemon Legends Arceus did it really sink in just how cucked this guy was for the longest time to the point where I now think he could qualify to be amidst the most mishandled Pokemon in franchise history. Like... How do you just introduce
literal canon god:tm: and relegate it to a limited-time event, only to then proceed to cancel said event (If the azure flute was considered too confusing for kids, then I am so curious to know how the hell the regi puzzle from RSE got past the cutting room floor)?! And then in the years after Gen 4's conclusion he just got nothing, with Conquest being the only real bright spot for Arceus for a good 12 years. We even eventually got Silvally and look how prominent it was throughout Generation 7! I just... ugh, I hate it, thank the lord for Legends.
To close things off I want to address one of the things people bring up about the gen 4 mythicals that in my opinion makes them vastly overrated: Their unique locations. It's a cool idea in theory giving them special areas you must traverse through to find them makes sense but... All of these areas are just basically just hallways leading up to chambers with nothing else going on with them? No puzzles, no wild Pokemon, just... nothing.
And if you wanna really talk about hallways, check out Seabreak Path:
Going back to Arceus getting fucked, the Hall of Origin might be the lamest of all:
I get this is the DS and there's only so much that can be done, but is this
really all they could think of? People complain about the Ultra Space locations in USUM being glorified corridors (and rightfully so), but even THOSE had more going on than these places. At least they could claim they had interesting visuals and sometimes had NPCs and items to find!