Before the low price, internet bikers pile on here, some perspective on Bosch is called for.
Bosch does not make any ebikes. They make and support the E parts of the bike for over 30 different manufacturers. Bosch has a well deserved reputation for both build quality and product support. Bosch e bike systems are not found only on the "mega expensive" bikes. They can be found on quite a few bikes is the mid range in terms of price as well as many of the top end bikes.
My own experience is on five different Bosch equipped ebikes, riding almost 13,000 miles in the last 18 months, without any problems in any electrical components. But that is just me.
Every dealer/service department I have spoken with has indicated fewer problems with their Bosch equipped bikes than any other. This does not mean that they are perfect and trouble free. They are fully supported though with a tech service line and a solid supply chain with any and all parts that may be needed to get any bike back up and running. Does any Alibaba bike offer this?
I know for a fact that Bosch has service training events in all major cities and many smaller ones to keep front line service techs up to speed and current on issues and fixes. If a particular dealer doesn't want to pay their service guy to spend a day learning about the ins and outs of servicing the bikes they sell, that is on them, not the bike manufacturer or Bosch.
It sounds like the dealer in the case of
@simonns was less than knowledgeable about the equipment he was selling and just resorted to the, "let's switch out this and see if it works" strategy. To give him credit, at least he was there trying to help and get the bike back on the road.
That is 100% more than one would ever get from a mail order outfit, sorting out issues on the phone or in a on line chat session, at best...and then sending a part in the mail and having to replace it yourself or pay a local bike shop to do it.