the game requires some real life financial investment to get your toon to a place that you will be happy with. What that is for each person is different.
Regarding farming adena, isle of souls and toi are the 2 main spots, both of which are timed instance zones. IoS 1 needs lvl 110-112 to be viable. IoS2 needs 112+ and better gear, ToI can be entered at 110, but it may be hard to kill there until 113-115 range depending on gear. Storm and Atelias can give decent adena. Spores/Ketra/Varka/Neutral Zone also give decent adena for party play. You also need prestige (monthly fee) for adena farming, and all party members require it for anyone to get the bonus.
As others have mentioned: 500b to 1trillion in gear is referenced as kind of the "normal" required budget to get a toon in mediocre shape. This would include artifacts +4, +7 cloak, +12 bloody r110, exalted, etc. this doesnt mean you will destroy everything, but its unfortunately the current state of the game and the content. If you consider it will take a year for a normal person to get 110-112 form 0, and adena is about $2/bil if u buy in game via ncoin, thats about 1-2 grand over a year. Not that bad in my opinion for a time waste hobby.
Now, why are people complaining?
Well, the pay2win model tends to not be very rewarding the past 6 months. The loot boxes have gone dry and many "spenders" are getting annoyed. People are openly complaining and multiple people have moved to Core for cheaper prices. Until NCsoft realistically looks at their price model, i doubt this changes.
Bots are always complained about. The in game afk macro makes this a mute point to me, but some people still think this is an issue. Please be aware though, many of the bot complaints are in reference to item farming (dragons, blood swords, olympiad, etc.). The people complaining about a bot in an xp spot are probably misinformed.
Update was very lackluster and missed any good content for the majority of players. This has caused people to be rather upset with the state/direction of the game.
i can go on, but ive typed alot. Lineage 2 isnt what it used to be. It is an idler more akin to a mobile game than a true MMO. However, if you understand what it is, spend responsibly, it can still be fun. You can earn adena in game, but the investment to get there may take some time to make back.
Answering what the cost of fun is for you, and if L2 is worth that cost, is for you to decide goodluck