Slightly exaggerated, but only slightly: Imagine you could only drink water 6 hours before you got thirsty, but if you drank too much you might pass out (at any point 2-6 hours from now) or if you drank too little you're doing major damage to your body.
Really the only way to manage with that kind of insulin is to live a very, very low carb lifestyle. Its doable, but it limits your variety and leaves you prone to other potential issues.
Whereas, if you use pens (or from a vial) with direct injection, you also take a long acting insulin (levemir, Lantus, etc) once or twice a day (they are supposed to last 24ish hours but often people find splitting their total daily dose up into morning and night has better results). And these insulins cost even more than the fast-acting stuff.