I understand your point, you can assign an action profile to a specific transaction type, so the validations will only apply to that transaction type, to me that's the only reason because an Action is "better".
This GAP can be solved using BRF+ or a custom table (customizing type, don't forget the changes should be transported like the changes on the action profile) on the BADI
The BADI method will be the best for what you pointed, encapsulation and performance, and of course SAP support during upgrades, don't forget the technology is important but how you implemented is the key to know if is encapsulated or optimal, the execution of the action will be less optimal because you must have more steps (schedule condition and action triggered N times) for other hand the BADI will be only triggered when save, and if you have the validation encapsulated on a FM or class you can share it between BADI and other objects, like WebUI events, other BADIs, BTE, etc.
But the most important reason to go for the BADI is: you won't be able to avoid the save trough the action, as we already discussed in your other post raise "do not save" within CRM Action
I'm not going to insist any more, from here the choice is yours 
Cheers!
BADI Fanatic