The $R_{\rm h}=ct$ Universe is a Friedmann-Robertson-Walker (FRW) cosmology which, like $Λ$CDM, assumes the presence of dark energy in addition to (baryonic and non-luminous) matter and radiation. Unlike $Λ$CDM, however, it is also constrained by the equation of state (EOS) $p=-ρ/3$, in terms of the total pressure $p$ and energy density $ρ$. One-on-one comparative tests between $R_{\rm h}=ct$ and $Λ$CDM have been carried out using over 14 different cosmological measurements and observations. In every case, the data have favoured $R_{\rm h}=ct$ over the standard model, with model selection tools yielding a likelihood $∼$$90- 95\%$ that the former is correct, versus only $∼$$5-10\%$ for the latter. In other words, the standard model without the EOS $p=-ρ/3$ does not appear to be the optimal description of nature. Yet in spite of these successes---or perhaps because of them---several concerns have been published recently regarding the fundamental basis of the theory itself. The latest paper on this subject even claims---quite remarkably---that $R_{\rm h}=ct$ is a vacuum solution, though quite evidently $ρ\not=0$. Here, we address these concerns and demonstrate that all criticisms leveled {\it thus far} against $R_{\rm h}=ct$, including the supposed vacuum condition, are unwarranted. They all appear to be based on incorrect assumptions or basic theoretical errors. Nevertheless, continued scrutiny such as this will be critical to establishing $R_{\rm h}=ct$ as the correct description of nature.