Is there ANY disadvantages of charging MBP 13" with a powerbank that outputs 10W? Device is Macbook Pro 2015 13". It has 74.9 Wh battery. Its original adapter is 60W. While charging the device will stay turned off.

If its allowed, I'll link an example powerbank. Its 25000mAh / 92.5Wh. Output (Max): USB 5V/2.1 A. 

I suppose from 0% to 100% would take 8-12 hours. 

Is there ANY disadvantages of charging MBP 13" with a powerbank that outputs 10W?  (Besides slow charging time :)

PS. I don't even know if this is possible.

For adapter, I'll use this and maybe this
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.