There are some things to consider:

## Prevalance of condition immunity
[Stunned][1] 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][2]

## 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 :)


  [1]: https://www.dndbeyond.com/monsters?filter-type=0&filter-search=&filter-cr-min=&filter-cr-max=&filter-armor-class-min=&filter-armor-class-max=&filter-average-hp-min=&filter-average-hp-max=&filter-is-legendary=&filter-has-lair=&filter-condition-immunity=14
  [2]: https://rpg.stackexchange.com/questions/128167/how-can-i-end-the-stunned-condition