Is there ANY disadvantages of charging MBP 13" with a powerbank that outputs 10W?
No. Slower charging is always healthier for the battery and the chemistry since it reduces heat. 

Lithium chemistry service life is severely degraded at high charging temperatures (hotter than any device would charge, but the higher the temp the fewer cycles you will get on average). 

As long as you can abide by slower charging, there is no other downside to your scenario.