Nuclear batteries were once used for pacemakers, but no longer.
They actually have some disadvantages:
Power output is constant, even when there is no load on the battery at all. Since the battery has to be big enough to meet its peak demand, a lot of the stored energy will be wasted. Also, the batteries have no real shelf life as such - they start draining as soon as they are made.
Power output drops over time. The battery has to be designed to meet peak load throughout its service life, so its initial output will be much higher than is usable. When the battery is no longer capable of meeting peak demand, it still has a lot of energy left in it.
Because of these factors, only a small fraction of the stored energy can usually be employed.
Converting radiation into usable energy entails more bulk and weight, components that can fail, and a loss of energy in the conversion.
Nuclear batteries cannot easily be recharged.
At the end of its service life, the battery must be disposed of or recycled. In many (not all) cases it is possible to recover the balance of the radioisotope and use it in a fresh battery. This isn't something you can do in your garage, however.
Not just anyone can be trusted to have a nuke battery, especially a large one. A strontium-90 battery will still put out a significant fraction of its original radiation a century after it is made. Such devices need to be tracked for a long time.
A large nuclear battery (or many small ones) can be very dangerous if it falls into ignorant, let alone malicious, hands.
Shielding is not usually an issue. There are isotopes available which require negligible shielding. However, they must be adequately packaged to resist damage. You don't want one of these things cracking when you drop it.
Nuke batteries generally have quite poor power-to-weight ratio, because power conversion is inefficient and packaging is heavy.
The main advantage of nuclear batteries is that they can serve for a long time without recharging (or any kind of maintenance). They are mostly used for providing power in remote places like spacecraft, weather stations or lighthouses.