Buying BTC through using dca is good and more convenient for investors, it aids an investor to control the volatility of BTC, and in my humble opinion it prevents naive investors from investing more than they can afford to lose in one lump sum purchase. Read more about dca in the link above.
These risks in decentralized exchanges are the same as the risk of sending BTC from your own non-custodial wallet to somebody else. If you use Binance 'fake' p2p for example, Binance in the case act as the intermediary and you hand over funds to them for the trade to be completed, that is why i call it 'fake' p2p, because there is a presence of a third party, do not think it is risk free too, there is also a possibility of a 'counterparty' risk, which may not only come from the 2nd party in the trade, but Binance themselves as they can confiscate your funds anytime and ask for more personal documents from you.

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In Decentralized exchanges there isn't any third party intermediary, you are sending funds directly to the 2nd party in the trade, that is exactly how the BTC network works, so you have to be careful and take responsibility as is expected of BTC users; before using decentralized you should read tutorials that will help you understand how to use them well. In the BTC network, lost funds are irreversible because the network does not work like a bank that is controlled by a central body, so decentralized exchanges not having a dedicated support team isn't a disadvantage because that's how the network operates, that is what makes it decentralized.