I understand using phone while it is being charged by a normal charger is not advised. What about using phone while it is being charged by a portable charger (powerbank)? I own a HTC one and i was told by a mobile technician HTC’s own portable charger only should be used for HTC one. How much truth is there to it as HTC powerbank is not available in my city and i am thinking of using Sony portable charger for charging my HTC One?
I use my phone constantly with a portable charger attached - to make phone calls, listen to music and watch video. I've never had an issue.
The HTC one charger is the same charger HTC has included with most of their phones for the last few years (5 volts, 1000 milliamps), you will be fine using it to charge any standard phone that uses USB. All USB devices use 5v, however high power devices such as tablets can require much more than 1000 milliamps, so don't use them.
There are little to no reasons for avoiding the use of a phone while charging. You'll be fine.
Powerbanks are just USB batteries, and since all USB devices use 5 volts, it will be safe. They are nearly all the same. Again, tablets may not be compatible with it though, because most tablets require a special high amperage charger.
The high amperage chargers will however charge your low amperage phone just fine.