Independent of your assets, that you submit, you should have less than 50% of rejections. Beginners can have higher rejection rates, experienced submitters should be below 10%, I guess. The 50% rejection rate seems to be an Adobe metric, it was once used as one of the conditions to get a free one-year subscription.
There is no warning about too many rejections, but having countless rejections will hurt your standing and after a while, you should learn from your rejections and submit only correct assets. You can be blocked for submitting an excessive number of assets that do not meet the Adobe quality standards.