Why is quantizing the free electromagnetic field in the Lorenz gauge more subtle than in the Coulomb gauge? Quantizing the free electromagnetic field in the Lorenz gauge, $\partial_\mu A^\mu=0$, is subtle. We must add a gauge-fixing term to the action so that $\pi^0$ does not vanish identically. Also, we cannot impose $\partial_\mu A^\mu=0$ directly as an operator equation because again the commutator relations cannot be satisfied even with $\pi^0\neq 0$. So we have to implement it via Gupta-Bleuler's suggestion.
But in the Coulomb gauge $A^0=\vec{\nabla}\cdot{\vec A}=0$, the quantization proceeds in a rather straightforward manner.
What is the root cause of this problem? Does the Lorenz gauge condition fail to remove all the gauge redundancies? Is a simple way to understand this?
 A: The thing about the Coulomb gauge is that, in vacuum, you get both $A_0 = 0$ and $\nabla \cdot \vec A = 0$, so it eliminates the $(A_0,\pi^0)$ d.o.f. right out of the gate - we just have $A_0 = 0,\pi^0 = 0$ consistently and only need to care about the spatial parts, and due to $\nabla\cdot \vec A= 0$, the whole thing is just a bunch of oscillators in Fourier space, where the condition $\vec p \cdot \vec A(\vec p) = 0$ then eliminates one of the three remaining degrees of freedom. Note that because the Coulomb gauge has broken Lorentz covariance anyway, we don't care that this elimination of d.o.f. is not stable under Lorentz transformations.
The Lorenz gauge, in contrast, is a Lorentz invariant gauge condition and hence we're not allowed to just "eliminate $A_0$" or something because what's $A_0$ in one frame is a linear combination of all the $A_\mu$ in another - the point of picking the Lorenz gauge is precisely to preserve Lorentz covariance. So in a way the "difficulty" isn't as much down to specifics of the gauge condition as it is to us trying to do things covariantly vs. non-covariantly, but either way the core difference is that we're not allowed to drop the 0-th d.o.f., which really make the whole thing much simpler in the Coulomb gauge.
Additionally, the Lorenz gauge indeed does not completely remove gauge freedom since it remains invariant under harmonic gauge functions, see this question and its answers.
