Outnumbered bonus for all eu is imperfect solution, but less worse than alternatives; A dynamic bonus amount per shard would only increase shard hopping, dropping outnumbered bonus would worsen region faction balance, and dropping 10% bonus would worsen participation and faction balance. What we have now works to a degree, only went to 30% because of patch.
Success or failure depends on how you define success or failure. Blizz probably consider it a success based on participation. They have the data and know how shards perform. I suspect they are satisfied with shards being variable balanced, but on average not too outnumbered. Our wm survey indicates players generally feel outnumbered half the time or less in eu. When very outnumbered, usually just an lfg group in your shard.