samsung galaxy s8+ was reset to factory state, could i get my data back?  

As far I know. The encryption keys are lost and no way to retrieve data without those keys.  

Unless you have a back either on the cloud or to another device, the data is gone.  

The data is there, but encrypted and the encryption keys are gone. On factory reset the userdata partition is not wiped, just the encryption keys are deleted.

Even if decrypting it now is almost impossible, I would suggest to create a backup of the chip at NAND protocol level.
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Apple passcode unlock + decrypted filesystem dump, Android user locks unlock + physical dump with decrypted userdata partition. We provide our services world-wide, but we reserve the right for choosing which tasks we take and which we deny! 

Thanks all