In my experience I heard that quite a lot, and I myself once think like that, mostly it's when people play them for the first time ever, and compare it with a more 'traditional' bass.
But it actually not the low end that is lacking, it's the rest of the frequency that stands out and can be heard much much more compare to ANY other Bass out there, and as the human ear hears the mids and highs more at close proximity (like standing or sitting next/in front of an amp), so when people try MTDs for the first time, that 'hi-fi' sound stands out IMO.
I myself thought that my former MTD needs some low boost, that's why most of the time I boost my lows and low mids on my onboard EQ, coz that's what I want to hear from my monitor and my amp, but one day my sound guy told me that he loved my MTD's sound coz he just left everything flat, and cut the low EQ on the board a little. That's new information for me, since then I never boost my Low Eq again, I set it flat and only boost the low mid around 25%, boost the lows only on my amp and my sound guy literally set everything flat on his end, he's one happy sound guy for sure, and when I sold that MTD to fund my custom build, he was unhappy and always asks when is my new one coming