There are some things to consider:
##Prevalance of condition immunity Stunned has quite a few monsters immune. Incapacitated has none. That initially makes monsters immune to the condition that normally wouldn't be.
##No means of ending the incapacitated condition There are some (very limited!) means of ending Stunned
##Makes things more dangerous The incapacitated condition can be delivered via a lot of other means and conditions. Wrapping that into stunned adds additional negatives besides action restrictions.
Giving attacks advantage and autofailing strength or dex saves increases damage/death risk. You have also removed movement as an option by moving to Stunned.
This sounds like what you're going for, but thought I'd spell it out :)