It's a little subjective though. Some people think that they can't get a warlord, others look at the current options and think "Cool, I can make my warlord in 5e". It's the same with the Ninja, some look at the options and think that the shadow-monk fits for their ninja, whereas others won't.
A little subjective, sure. But it is objectively a huge difference of severity, at least.
Every "warlord" option in the game is a sliver of warlord on top of a big meaty helping of an entirely different concept that has little, if anything, to do with the warlord.
The Bard is only vaguely similar in that both so support. Sorry, but that is about as much a warlord option as warlocks are a Druid option. The Battle master is about as much a warlord as the Eldritch Knight is a wizard.
The ninja Monk is a ninja, sans, what, two abilities? Maybe?
I know you haven't been making that arguement, but I don't feel like multiquoting, so let's review that.
1. Water walking. Nope. Level 9 Unarmored Movement.
2. Change Tracks: lets break it down
2.a. Making it impossible to read your tracks. Level 3, pass without a trace.
2.b. Tricking someone into thinking you went a different direction. Minor Illusion and skills. With MI, you do it better than others, but others can also do it. As it should be. 1e didn't even properly have skills, so it should be no surprise that skills cover some of what a class did in 1e. If there were a tactics skill in 5e, warlord fans would expect some of the warlord's shtick to live there, and warlords to have feature that synergies well with it.
2c. I'll give that this one could easily have been added to the Shadow Monk without beefing up the subclass. 1 ki to extend and expand minor Illusion for a few specific purposes, including changing tracks.
3. Walking through walls. This one I kinda grant, although you do get 60ft teleport at level 6. I only kinda grant it because you can't teleport if you can't see the destination. It should include the option to instead gain a phasing movement of 30ft for a turn, as a bonus action. *
4. Holding breath. I agree. This should be part of the Monk. Monks should be better at holding their breath.
5. Create illusions. Lvl 3, Shadow arts.
It doesn't even just imply strongly that it is a ninja, but literally says that it is a ninja. It's missing two things, at most. One of which it replaces with something that is along the same theme, but arguably stronger.
But again, I fully support an assassin class, just like I support a Captain (warlord) class, a warden, a true summoner, a shaman/witch, an "Avenger"**, and the Artificer and Mystic.
I'm just saying, the ninja is vastly better represented in 5e than the Warlord. A ninja player can "ninja" every turn, most parts of exploration, and many parts of a social encounter. The various "warlord" options can't. They can be a fighter 80% of the time, and warlord every now and again. Or replace fighter with rogue or paladin. It's not the same, at all.
*anyone else think Shadow step is a big power increase that would work better if you get minor teleportation at level 3, one or two fewer spells at that level, and then upgrade to full Shadow step and get those delayed spells at level 6? Also, at will bonus action 60ft teleport and advantage on an attack...dude. That is a lot. I'd rather have advantage on Stealth, and gain a teleportation movement for a turn, so I can teleport my speed, along with increased speed in dim light or darkness.
**which would be a wis based unarmored holy warrior whose hand is guided by faith, and thus adds wisdom to AC and damage, possibly attack. play like a Monk/paladin in some ways, with a few spells it casts without spell slots, SCAG cantrips instead of Smite, gets evasion, etc.