I've used an Optimate on my 2017 GSA since new. It is garaged for several months at a time. It is connected to an SAE pigtail directly off the battery. It is microprocessor controlled and checks the battery voltage every half hour or so by applying a small load, for accuracy. If the battery voltage is within range it does nothing. If the battery voltage has slipped it applies a low amperage charge until target voltage is met.
The Optimate costs more than the Battery Tender but connected to a $26k bike, I went with the premium. That said I've never heard of Battery Tender brand causing problems and believe me, if one of these units caught fire or blew the electronics on a BMW bike, the event would be broadcast from the tallest tree.
Thats a 2 thumbs up for Optimate smart battery chargers. They are the gold standard for battery maintenance, do not buy or use a "trickle charger", use some type of "smart charger", it will monitor, charge, de-sulfate as required. You will never buy another battery !
I would expect to properly recharge a battery we would use a "battery charger" and not a " battery tender" each having a distinct and dedicated purpose. The first is to bring a "dead" battery back into operating condition and uses 3-6-12-25 amps to boost or jump the battery up over a short period of time. The maintainers are to keep a charged battery in optimum range, preventing unnecessary wear on the bikes own recharge system. Some newer batteries actually call for lower amps and longer charge cycles to prevent damaging the cells, Gas Mat types, and some of the newer Lithium types can be especially sensitive to large amp pushes. The presumption above is that they recommend disconnecting the battery from the bike when reviving a dead battery and assuming that the battery can take a large amp boost. The newer bikes have lots of low voltage sensors and light now and may be prone to over current damage. That just my hat in the ring thing.