I think that's fairish comment. That said, I do also think that another problem that some people have with Milhaud is the sheer prolixity of his output; "Where does one start?" is a plaint I've heard more than once. My best suggestion is with the songs, quartets and symphonies, although with the best will in the world even I have no small amount of difficulty in figuring out what he wrote so much music; it seems as though it simply fell out of him - an impression I also get with much of the output of the even more prolific Niels Viggo Bentzon (which likewise includes some music well worth getting to know but who one pianist I know nicknames "the Danish bacon factory"). Milhaud strikes me as one who prized French elegance above certain other virtues (although I well realise as I say so that this is the most simplistic of generalisations) and, as such, comes across as a rather larger-scale and at times somewhat more serious-minded Jean Françaix.
I've been studying works by Bentzon for some time and, while he's certainly been prolific over the years, his status as a "prolific composer" seems to be discussed more than the actual works that make up his catalog. It is fairly well known that his oeuvre exceeded 600, but an incredibly small percentage of those pieces have actually been explored beyond their initial performances when Bentzon originally presented them. Any question of where to start is quickly stymied by the fact that there are less Bentzon scores and recordings than there are scores and recordings by numerous composers who've not exceeded 100 opus numbers. Aside from the Tempered Piano volumes which he recorded on his own, I have only heard 2 of the numerous piano sonatas that he wrote in the last 60% of his years as a composer, and those were recorded by him as well. With his prolific amount of music, it's not even a matter of "what should I start with" as much as "what
can I start with, if anything." Other prolific composers are far more obscured than he is. Jan Van Dijk from the Netherlands has doubled the amount of total compositions of Bentzon (yes, I mean 1200 opus numbers) and NONE of his work is available on recording. I've heard bits and pieces of his piano sonatinas and it absolutely makes no sense why this music has to be so
completely unknown. I guess it's a widespread case of "I don't know where to start, so f**k it, I'll just ignore him and listen to another version of the Rach 3 or a new Beethoven cycle." All facetiousness aside, I'm more sure that Danish, Belgian, and Dutch composers like these generally get the shaft in the public eye because they come from smaller, more low-key countries that lack the historical clout of Germany, France, England, and the US and similarly lack the traditional "exoticisms" of Russia, Poland, Hungary, etc... (most of which are known primarily from their historical associations with the first group of countries. I mean, look at a composer like Per Norgard (another heavy-hitting Dane). He's been composing astounding music for decades (his pieces are often placed on discs alongside works by Ligeti, Xenakis, and Penderecki) and he's still almost completely unknown.
With Milhaud, I would recommend starting with a sampling of all of his different styles. The quartets are probably the best place to find his greatest musical strengths, and the rest of his chamber output is not far behind. Like I stated before, good luck actually finding recordings of the music though. Milhaud, like Bentzon, is definitely amongst numerous composers whose profundity means almost nil in the classical recording/performance world (Absil, Tansman, Jan van Dijk, Badings, Mihalovici, etc...). Surprisingly, a ton of Martinu's music has been recorded, though he's also prolific and often mentioned in the same discussions as the guys I mentioned. I think my jaw would drop to the floor if I heard about a string quartet putting on a performance of any of the numerous works by these composers that haven't been touched in years. Bentzon wrote 14 string quartets and a grand total of ZERO are feasibly available as recordings (I think the 6th quartet was recorded on a Danish LP several years ago). I've been studying Absil's 4 string quartets (imagine a Belgian cross-section between the best things found in Bartok, Tansman, and Milhaud) and I'm pretty sure I'm one of the only people who's ever purchased those scores. Milhaud's quartets have been recorded twice but both disc sets are downright impossible to find unless you're lucky enough to spot one in the school library.