How accurate are COP areas to 2005-2006? I mostly refer to the difficulty of boss fights.

If u remember the un-nerfed fights and loved them in retail, I'm sorry to say most of the fights are kinda easy on nasomi. ex:

- no need for i wanna say yellow liquid on mammets
- no salt thing needed for snoll fight. also i dont think it explodes?
- Alot of dum crashes. the tele pads in any promy should be used w/ caution. Shit what uses the mob that crashes everything.... the thinker with trinity tap?

Given all that and the tenzen techno dance palace, its beatable and u can get ur ring ^^

Fights that are trivial, usually due to no AI and no proper fight mechanics:

3 Promyvions
Final bosses of Promyvion Vahzl
Mithran tracker fight
Snell fight
All airship fights
All fights in sea

Several bosses don't have correct behavior, but can be hard due to being overtuned (Diabolos, Vahzl mini bosses).

A few missions have bugs that you have to fight through

I'm sure I have missed some things, but that should give you an idea.

In case it helps, I grabbed a copy of the "Nasomi CoP Differences and Tips" article from the Naswiki when it was up last time:

Nasomi CoP Mission Differences and Tips (Part 1) 
(Thanks to Meanthras folks:  @Lyrisse for the original write-up and @Nireya for additional coments)

CoP 1-3 Promyvions: Wait for Strays' bodies to disappear before using teleporters to avoid blackscreen. 
Mobs may follow you between floors. Bosses do not use abilities, except Mea which has Throat Stab+Hate Reset.

CoP 2-3 Aqueducts: Fomors aggro sound and HP and link, but are not partied. 
Have one person run to Minotaur and it will draw party/alliance in. 
Can skip key farming by using Thief's Tools (skeleton keys don't work). 

CoP 2-5 Mammets:  Mobs may follow you between islands. Mammets sleepable and easy to sleep. 
Yellow Liquid does not work.

CoP 3-3 The Road Forks: Antlion solo-able by most jobs@60. Click Loose Sand again to be teleported to top of mountain - no climbing.

CoP 3-5 Diabolos: Does not use TP moves or Nightmare. Tiles will drop, but is not visible. 
Diabolos will cast spells and draw-in. Can be silenced. Can be Carby pulled or BST jug pet pulled to top platform.

Tile placement:
XXDXX - D = Diabolos
OXXOO - OO = Suggested Melee Standing Location
XXXXX 
XOXXX
XXOOX - OO = Suggested Mage Standing Location

CoP 4-2 Ouryu: Mobs may follow you between islands. Mistmelt does not work. 
Working strategy is 1-2x BLM freeze and sleep OR SMN with Ramuh. Ouryu may not come down once flying. 
If he does, he uses Invincible when he lands and at start of fight. 

CoP 4-3 Sacrarium: Fomors aggro sound and HP and link, but are not partied. Maze changes randomly and not necessarily based on game day. 
 THF hide can de-aggro and despawn Fomors, even for HP aggro.  DD/NIN recommended to absorb Fomor TP moves.

CoP 5-2 Promyvion - Vahzl: Wait for Strays' bodies to disappear before using teleporters to avoid blackscreen. 
Mobs may follow you between floors. Bosses do not use abilities, except Mea Boss which has Throat Stab+Hate Reset. 
BCNM bosses do not link and can be fought one at a time.

CoP 5-3 Three Paths - Tenzen's Path: Zone is glitched causing mobs and allies to be invisible at times. 
Mobs can also aggro through doors. Run around until you can see.

CoP 5-3 Three Paths - Ulmia's Path: Mithra Trackers can be slept. Snoll easy. 

CoP 5-3 Three Paths - Louverance's Path: Gamble or use Snow Lily to teleport to Mine Shaft #2716 (gamble recommended). 
BCNM fight works as intended - mobs do not sleep, but are susceptible to bind and gravity. Highly recommend RDM with 2HR Bind.

CoP 6-4 One to be Feared (Airship Fight): Easy fight. Members of fight may DC when using doors. 

CoP 7-5 The Warrior's Path: Tarus are not present for fight, making the fight trivial.

CoP 8-1 Sea Towers: Aerns don't 2HR. Not all PT members may get the kills counted, so they may pop more Aerns. 
To prevent having to refight the Aerns pulling them 20-30 yalms away from the ??? then killing will prevent that bug.

CoP 8-2 Escort: Escort NPC does not function. Only required to run up. Can be skipped by using the central teleporter.

CoP 8-3 Tower Climb & Pots: Can pull one at a time. Cannot be silenced. Ice spikes cannot be dispelled.

CoP 8-4 Final Battle: NPCs don't work. No shields as Prishe can't remove them. No Meteor. Casts Comet and can be silenced.

I'll add some notes for CoP 5-2, since it's a whole #JustNasomiThings funfest sometimes:

* One of the bosses (leggyboi one, I think?) does a TP move that can crash the zone if not stunned (Trinary Tap, I think?). You can logout/logon right before engaging the floor bosses if you're paranoid about it, since it'll place you back in at that point if the zone does crash.

* Similar to above, when you get to the last floor, have EVERYONE zone into the Spire first before running to the boss. It's not terribly far out of the way, and it'll place you at the spire zone line if that boss crashes the zone.

* CHECK YOUR LEVELS BEFORE POPPING FLOOR BOSSES - the level cap doesn't always apply correctly, and if someone comes in uncapped it can cause all kinds of funkiness. Just logoff/logon to fix (sometimes takes a try or two). Do everyone a favor and take off /anon .

* Sometimes the bosses just spawn funky and end up way harder than normal, even with everyone at the proper level. Try popping it and then have everyone logout/force quit and try again.

For Tenzen's path, my recommendation is to keep an eye out for someone offering a tractor to the end >.> Pso'Xja is pretty glitchy in general but Tenzen's Path is a whole 'nother level. Having your whole party/ally get wrecked by Cryptonberries that no one can see sucks (especially when it's a NIN mob sitting somewhere in the room not moving). That said, it's one of the few "Hard Mode" things to do here, so it's fun in its own special masochistic kinda way

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I'd put cool pics of my achievements/items here, if they were anything interesting. How about "I pooped today! \o/"