There are no drawbacks of using Power Banks with your smartphones. But you must consider these point before buying any of them.
1. Brand - Always buy a branded power bank which is compatible with your smartphone otherwise it may led to damaged battery or low battery backup. I will not prefer you to buy from any online store other than official websites because they are the only one which sells original stuff. As we know products on Amazon and Ebay are sold by local seller, many of them ship fake products which are 100% identical to original ones. Most of the cases of fraud are with the power banks because of wide range of manufacturers. So either buy power bank from showroom or go for official site. I will prefer you to buy OnePlus Power Bank it gives very good battery backup. You can also consider Xiaomi Power Bank but before buying watch about How to identify fake Xiaomi Power Bank on Youtube.
2. Ports - Many power banks are reported to drain battery while USB cable remains connected to it. So you must consider this. If this is an issue then there is problem with the data cable and it can harm your device as well. You can avoid this by using original data cable which came with your smartphone.
3. Output - Focus on the output of that power bank it must fulfill the requirements of your smartphone otherwise it will not be able to juice it up.
Power Banks also comes with batteries which comes with a limited charging cycle, so put that in mind. After charging cycles of about 400-500 your power bank will loose its power. You will need to change battery if available or buy another one.
I am currently pursuing High School and planning to pursue career in Computer. Currently, I work on one of my blog - JTech Articles.