I think it really pays to go with an e-bike from an actual bicycle company. The Trek TQ system for example is submersible (IP67) and has a 2-year warranty, same as Bosch.
Is it safe to generalize like that? IP67 is a certification criteria that says that it must withstand conditions for certain amount of time during the test. But it doesn't say anything about doing it twice or how long you must wait between tries.
For electronics it sometimes achieved by gel that can hold liquid for 30 min before water penetrates. It may eventually get dry again, but that is not covered by certification.
Another point is that testing is done in distilled water, any impurities like oils can increase its penetrating ability and reduce the time.
To me it looks like it's all down to actual manufacturer and item. IP rating is better than "splashproof" but still very vague.
