While respecting your background and experience, I can think of no technical reasons for the statement you make above. I think that there are two types of low-current chargers - a trickle charger, and a 'maintenance' charger.Qzyn said:
The trickle charger has no cut-off facility and will continue to deliver current even when the battery is fully charged, which can result in 'boiling'. Not literally, but that's the apppearance from the bubbling. So it needs to be monitored.
The maintenance charger is designed for non-attended operation, eg, on boats & motorbikes, which get laid up. When the battery voltage is sufficiently high (fully charged) the charger switches itself off. This is perfectly safe to be left connected for long periods with no damage to the battery.