I mean you’re describing a huge variety of products with that one phrase, and I welcome the question - But the intelligent parts of a smart charger are components that can break or die, particularly if you’re constantly stressing it with being on and checking to add voltage/cycling. One part of the circuit on these dies out and other protections stop working too. Batteries are pretty unstable once that starts happening and then fire from heat or too much voltage becomes pretty easy to happen.
The battery itself is built to have some protections, too, but if you google “battery charger fire” you’ll find hundreds of images and news stories.
Just sayin’ - Use a charger for a battery til it’s charged and unplug. Smart phones and tablets seem to be more robust so I trust them to stay plugged in with their charging cable.