Helmets without any reflective elements are common in road and aero designs, where clean aesthetics and weight savings take priority. If you mostly ride during daylight hours or on well-lit roads, the lack of reflective material isn't a major drawback.
However, if you ever ride in low light—early mornings, evenings, or overcast days—you'll have no passive visibility from your helmet. That means cars won't catch a glint of light from behind or the side. You'll need to rely entirely on bike-mounted lights and reflective clothing to be seen.
The key trade-off is convenience versus style. Reflective elements work automatically with no batteries or effort, while a helmet without them requires you to be more intentional about your visibility setup. If you're a dedicated fair-weather, daytime road rider, a helmet with no reflective elements is perfectly reasonable—just don't assume a bright-colored shell alone makes you visible after dark.