Phantom is simply a mess. It's a mesh of fine ideas that ultimately is not executed well and ends up with a lot of wasted potential. Let's dive down deeper to see how Phantom ended up.
This review contains minor spoilers after the "Read More" mark.
Phantom consists of 3 main arcs, each separated by a recap episode, and each focusing on 3 different characters, Ein, Zwei and Drei, although with Zwei being an important character throughout. The story starts of with an interesting premise of an amnesiac man being manipulated by a shady organization.
However this thread is very quickly resolved when he learns of his identity 3 episodes in. And Zwei, oh Zwei isn't known for making rational or even normal decisions. I mean, how else did he get himself stuck in this situation am I right? Several times in the series I really question his true motivations or even reasons for carrying out certain actions, not because of some mysterious plot thread or engaging character behavior, but because of shoddy character writing.
The other main characters aren't any better in this regard. Ein is your standard emotionless girl who finds emotion through main protagonist stereotype and is absolutely dependent on someone else, be it Zwei or Evil Doctor person to live. Her character development is cliche as all hell, and ultimately she isn't much likable as a result.
Drei on the other hand is more entertaining. She is a lot more enjoyable than the other 2 characters when it comes to her scenes, although her character motivations are just terrible. I won't go much into her reasons for being in the plot, but it feels so shoved in and terribly written that I couldn't garner any sympathy for her plight.
The other side characters are pretty forgettable. The main antagonist is an evil doctor whose name I forgot and is always one step ahead of the protagonists but has nothing substantial about him. He's evil and depraved and that pretty much sums up his entire character. Near the end he becomes a lot more theatrical and a lot more entertaining but that still doesn't redeem any of the previous moments involving his character.
There's also the issue of the overall plot. The plot is competent enough, but the characters and the pacing drag it down significantly. The pacing in particular ratchets up so quickly near each arc's end that the audience would be left confused and tired by it's conclusion. The third arc manages it all best, where Zwei and Ein are more likable/less dumb and Drei always makes for interesting encounters.
The series has very bad problems managing its themes/messages. One of the main themes the series attempts to showcase is that a name isn't everything and one should live to be oneself. This theme is delivered in the first arc when Zwei told Ein to ditch her old name and live anew, saying that she shouldn't be retrained by a name. Ironically afterwards he starts calling her Eren and expects her to abide by and live by that arbitrary name he gave her. Ein even calls him out on this but the show just ignores this and thinks it has written something profound or meaningful or something. It's ridiculous.
There's also the ending which is just terrible. It differs from the original visual novel by purposely altering it slightly to make it a bad ending but doing so completely defeats the purpose of the protagonists' journey and is also inconsistent with the previous parts of the anime where they could eat up bullets without needing too much medical attention. It's just bad.
There's a lot more I could go on about but I should just stop here. Phantom isn't worth your time.