This is slightly subjective but important nevertheless. Not all investigations end up proving you guilty but as a customer if you have a choice as well be cautious. Avoid the DP if there are too many negative vibes on social media, chat forums as well as constantly being pulled by the central DPs and the regulator. That is a sign of laxity in process and that is the last thing you expect from your DP. When in doubt, think with your feet.
Demat account offers the facility to hold a broad spectrum of financial investments in a single account including equities, bonds, ETFs and even mutual funds. This enables you to control all your investments with a single login. Most DPs also offer you analytics that you can use to fine tune your holdings and transaction trail analysis.