I've had the pleasure of upgrading from a 2015 to a 2017. The battery life on the latter is definitely worse, no matter what the official specs say.
I'd guesstimate I get at most 3 hours on the 2017 before the battery level gets low enough I have to start planning where I'll next plug in. Not ideal.
I suspect that Apple will be seeing a lot of 2015 MBPs brought in for battery replacements...