If you're using a Nexus 5X or 6P on Android 6, then probably yes.
Android 5 has full disk encryption but was disabled by default on the Nexus 5 (and most other phones) due to performance issues.
Do the Nexus 5X or 6P finally have hardware encryption support ?
I didn't notice any mention of those, specially because they failed miserably last year parading full disk encryption on Nexus 6 and got bitten when reviews showed the performance degradation and lack of hardware encryption.
It's a good question, I've looked and the Qualcomm Snapdragon 810 has the support needed, but I don't know if it'll be enabled by default or not in Android 6.
If it's not, then Google are starting to take the piss..
Otherwise the data is still there in the plain, accessible by anyone who can read the flash.