I remember Amanda Johnson saying if you owned 20% of the masternodes you can track 0.004% of the transactions. Masternodes doesn't do the Tx it simply connects those who want to mix. They have lots of improvements done and PrivateSend goes through multiple mixing rounds. It's economic suicide to use masternodes to attack privacy with a masternode being priced at 1000 Dash.

https://www.dashforcenews.com/privatesend-best-practices/