i) The most interesting, albeit abstruse, objection to the eternal subordination of the Son is the claim that this creates two wills in the Godhead, which is said to be antithetical to the unity and/or unicity of God. Mark Jones has been stressing this objection, although, ironically, I think he's taking his cue from Arminian Tom McCall.
ii) Although I don't subscribe to either eternal subordination or eternal generation, the question of whether two wills in the Godhead is incompatible with the unity and/or unicity of God is interesting in its own right.
iii) One problem I have with Jones is that he contents himself with expounding traditional distinctions, then leaves it at that. But if the truth of tradition is at issue, then that begs the question.
Mind you, there's nothing wrong with taking theological tradition as a starting point in some settings. That can be appropriate and necessary. Every ordination exam can't reinvent the wheel. The examiners are entitled to take for granted the truth of their creeds and confessions. They are entitled to treat these positions a settled doctrine.
However, that's not an absolute starting point. Belief in tradition ought to be justified belief. That preliminary step needs to be taken. Indeed, given conflicting theological traditions, we can hardly avoid the task of scrutinizing tradition.
iv) I'd add that this is a very recondite debate. Exegetical theology won't settle the question of whether there are, or can be, two wills in the Godhead. That's a question for philosophical theologians to hash out. Jones approaches this from the standpoint of historical theology, which is fine up to a point, but that's mainly useful in explicating what the position is, not in evaluating the merits of the position.
v) Divergent wills in the Godhead are incompatible with the unity and/or unicity of the God. But on the face of it, it's hard to see how one and the same God can accommodate three persons, but not two wills. Is a person less than a will? Isn't the will as aspect of a person?
vi) Perhaps we could show the compatibility of two wills in the Godhead by using something comparable that's easier to illustrate. Take mental states or viewpoints. Consider that in relation to diachronic identity.
Take my high school graduation. There's how I view that in advance. Then there's how I view that after the fact.
My prospective mental state or viewpoint is different than my retrospective mental state and viewpoint. Moreover, even in hindsight, there can be additional differences in my respective mental states and viewpoints. How I view my high school graduation a month later, year later, 5 years later, 10 years later, 20 years later, 40 years later. I look back on the event from a different perspective given where I am in life.
Is this one or many? In one respect, it's many viewpoints and mental states. In another respect, it's one and the same agent whose the source, possessor, or bearer of these viewpoints and mental states. Different viewpoints in relation to the event–differing perspectives on the same event, but the same in relation to the viewer. In each case, that's how I look back on my high school graduation (or look ahead, as the case may be). The same first-person perspective.
vii) Perhaps someone might object that diachronic identity is a poor analogy for a timeless God. God doesn't have shifting mental states.
But I don't think that's a problem. If identity-in-difference is coherent in the greater case of a changing agent, surely it's coherent in the lesser case of a changeless agent.