1) Ad fraud. DSPs and SSPs cannot always check the reliability of all traffic sources, which may result in bot traffic.
2) Brand safety issues. When you buy ads through RTB auctions, there's a chance that your ad might be shown on an unreliable resource. This can be mitigated by blacklisting bad quality websites.
3) Mark-up fees. Advertisers should be aware of the fees of different adtech platforms, including DSP and Agency Trading Desks.