Been through one several times this winter. I don't like the soap, but it beats the hell outta me doing it in 35F and rainy weather. Never noticed a leak. As to the soap eating the seal??? if that happens, BMw better step up. That would be absurd.
Pretty soon I'll have my external hot water hose bib plumbed and I'll hand wash from them on out.
'08 Alpine white E92 M3 with DTC--Sold
'12 Alpine white E92 335 Xdrive with slush box
'07 Summit white Chevy 2500 HD Duramax/Allison
'12 Alpine white Z4 35is via EuroDelivery June 4th