In name and power, yes, but what does being a Master really mean?
When I think about Aqua, Eraqus, Xehanort, and Riku, I see characters who are pretty confident in their abilities and their emotions/alignment (something Riku recently attained as a character, talk about development)
Terra is not one of these people. Neither was Sora, maybe not even Ven, or Roxas (as of 3D). Based on his convo with Xehanort inside his heart/body, I think Terra is on his way to becoming more 'stable', but he was far from 'stable' at the end of BbS.
I'd say that is the main gist about it. All those who have
officially gained the title out of the extended main original cast are the people who have a certain control and dominion over their own emotions and thus by extension over both the light and darkness residing in their hearts.
This of course does not mean as many people apparently seem to misunderstand that one should suppress/ignore his/her emotions. It only means to have a certain control and not let these emotions rule you by overly indulging in them.
Terra is certainly someone who acts more on impulse and let's his worries get the better of him while Sora is firmly on the "jump in, think later"-train acting on things rashly as well.
Ventus' case is a little blurry since he was not only considered too young but there was also the issue with Eraqus knowing about him being the potential ingredient of a weapon of doom. Mentally I'd place Ventus somewhere between Terra and Aqua on the spectrum as during the first half of BBS he certainly also acted somewhat impulsive as well as his overreliance on his (idealized) friendship with Aqua and Terra, but by the latter half he was clearly more focused and "stable".
For someone who was thoroughly broken and had much of suffering to endure Ven developed rather well, but certainly not yet true Master level.
Roxas is arguably a very clear case mainly because despite any physical appearance he's little more than a year old and thus simply has not the experience and "education" needed to reach a state like Aqua, Eraqus, Xehanort or Riku have.
Much of his life being rather traumatic certainly also doesn't help in this regard.
In that way, Eraqus wasn't technically wrong in saying Terra wasn't ready to be a Master, but he really sent Terra down the wrong path of figuring himself out. Terra, alignment wise, is like Riku. Darkness isn't necessarily bad, and it can be used for good if kept on a tight leash and if its user can maintain the proper balance or keep it secured and locked up tight like Riku. Eraqus insisting that any use of darkness is bad and light is the only way only made Terra more confused and hindered him.
In name, Terra can be a Master all he likes, but in my mind, he's not in Aqua or Riku's league. Yet.
Indeed, Eraqus quite probably
means the right thing but he totally fails in adequately delivering it. That Darkness
can be used for good if kept on a tight leash and kept secure is practically the whole crux of it and the
true way to properly handle it (not the bullshit Xehanort and Darkness fanboys are spreading) but Eraqus goes too far by preaching to actively suppress and hate it.
He does realize this error somewhat directly after his fight with Terra, but by then it's completely too late.
Going off on a little tangent, I'd say that both Eraqus and Xehanort are/were the wrong
type of teacher for someone like Terra.
