What evidence do you have to support these claims? I've been charging batteries in parallel, without any problems, however, only using a single charger.
As I understand it, as the lead acid battery nears a complete charge, its terminal voltage increases. This increase reduces the amount of current that battery will take, assuming of course, the charger is voltage regulated. As different batteries in the parallel bank reach full charge, batteries with less charge end up getting more current until they all equalize at whatever voltage limit the charger is set to. I can see where float charging with parallel chargers might be a problem. I can build up a comparator circuit that will disconnect the extra charger once the terminal voltage reaches a certain point (2.4v/cell for example) and let the single charger carry the bank to completion.
Of course, I can see there being issue if one of the batteries is damaged or overheated, and is unable to equalize its voltage with the other batteries - then it will be subjected to a large current as the charger is trying to raise its voltage. However, I'm not looking for a turn key automatic system here. I just want something to fast charge a battery bank for a few hours while I have access to ac power.