Honestly, it's a pretty significant drawback. A helmet without any fit adjustment system relies solely on picking the right size and the foam liner compressing around your head. That makes it really hard to get a snug, secure fit — and a loose helmet can shift or even come off during a crash, which defeats the whole purpose of wearing one.
The problem gets worse in real-world use. Your head shape might not match the helmet's internal shape, and without a dial or adjustment mechanism, there's no way to fine-tune it. You also can't easily account for wearing a beanie or different goggle straps thicknesses underneath.
These no-adjustment helmets are typically found on the cheapest models or rental fleets. If your budget is tight, look for at least an interchangeable pad system — but ideally, spend a little more for a dial-based system (like BOA). It lets you micro-adjust the fit with one hand, even with gloves on, and keeps the helmet properly positioned where it matters most.