Does a power bank create problem to a smartphone's battery?

As long as my knowledge goes, using a power bank should not have any issues on your battery life or its performance. Only point you need to note is to check whether the power bank you choose is compatible with your device.
Following a few basic precautions may be helpful.

1. Don't go for unbranded power banks
Stay away from cheaper ones. Those may be sub standard. Always opt for a branded power bank from a reputed brand and proper warranty.

2. Check connectors
Make sure the power bank features original connectors. Using the connectors that are not suitable for your phone may damage your charging point (cum USB connector). Also pay attention to the current being drawn is as per your device requirements.

3. Check features
A good power bank will always have additional features. Please check whether it has temperature protection, over charge protection, over protection and short so circuit protection. These are essential for safeguarding device against unforeseen fluctuations.

4. Pay attention to quality standards
Make sure the power bank meets the necessary quality requirements of your region. It is extremely important as it would be guiding factor in deciding the compatibility of your device device to your power bank.

In theory power bank should not cause any issues at all. Most of the powerbanks only support the power supply from unidirection. So while they are being charged you should make sure that they are properly charged. Avoid overcharging them as it reduces their capacity over a period of time.

If possible make sure that you use the reputed power banks. And use these devices only during the period of warranty. Avoid using them outside the warranty for the phone or tab.

If you find the power bank not charging the device for longer time then check both device and the power bank.

Power draining is a sign that the power bank is in need of replacement. So if you see any such signs, then change the unit. Do note that depending on warranty period power banks will slowly fade away with storage.

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.

Regards,
Joy Joon