There's lots of posts all about this, basically problems might occur if the tread depth is too different between tires. Everything from abs, traction, excessive diff wear, and so on.
Note though that the summer tire options for the X1 and all sorts of other cars all have slightly different diameter between the front and rear wheels _stocl_ so some difference is likely fine. Reports from 1%-5% variance seems okay for some situations, most try to stick to 2%-3%.
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