The point of the exercise is independence.
When you are deciding to purchase a solar or dynamo hand crank device, such as a flashlight or radio, you want to have a solid product that will work when you need it, even years from now. These self-powered products are almost always marketed as never needing batteries, but this is only partially true.
Is there a battery inside?
Usually yes.
In order for a radio or flashlight to operate, it needs some sort of reservoir of power to draw from. So, you may not need to actually put any batteries into the radio or flashlight, because there is already one inside. This internal battery is what the solar or crank charging system is connected to. Hence, free of batteries, but not battery-free.
The question now becomes, is this a reliable solution?
To answer this, we need to look closely at the exact nature of the internal battery pack. It is usually something like the battery pack you find in your cordless phone, and what you want to see stamped on the side of it is "NiMh". This is the chemistry that is best served in these products. Cheap. Easily maintained. Replacements are easy to find. Lithium is also a good choice, but are not very common in unregulated charging systems for cost & safety reasons. If the stamp says "NiCd", put the product back, and step away. NiCd is an old chemistry, has a memory effect, and contains toxic heavy metals that you do not want to contribute to the environment.
Will the internal battery ever fail?
Yes, like all batteries inevitably will. But if you choose a good brand that doesn't cut corners, that won't be the case for many years. You can also prolong the life of the internal battery by simply using the device and keeping it charged. If the flashlight or radio comes with a charger, or now more commonly a USB cable, just plug it in for an hour or two every six months to give it a full charge.
Are there any devices that don't use any batteries at all?
As a matter of fact there are. At least in the flashlight realm.
Applied Innovative Technologies, makers of the original magnetic shake flashlight and the dynamo versions: the SL1 & CL1, manufactures these flashlights without any internal batteries at all.
How do they do it?
They install an ultra-capacitor instead of a battery. Unlike a battery, an ultra-capacitor has no finite lifetime, and will simply work like any other electronic component. While they don't hold a charge like a battery, they can be used over & over without loss in performance.
Now you know. Choose a well known brand that uses NiMh battery packs in their rechargeable products. If short-term emergency lighting is the goal, look to the new capacitor-based flashlights.