I think both DPs and AMCs are same w.r.t safety and security of investments. And all these being connected with technology, and not physical, the possibility of fraud happening at one level or another always exists, and I AFAIK, I haven’t come across any theft of data which has resulted in the loss for investors, apart from the glitches while making transactions, which are common.
I would worry, if a breach happens and data is stolen, which results in unaccounted units, ambiguous statements, and I guess one solution here can be, to maintain our own records with statements downloaded regularly, and checking regularly if all the transactions are correct or not.
Been investing for some years both through AMCs and demat, never had any suspicion, nor the worry.
If you are really concerned, use a basic phone with no internet connection for all the financial transactions, so that we can eliminate the threat of intrusions through net.