When extending, sticky pistons act exactly as regular pistons, pushing up to 12 blocks. Although, when retracting, sticky pistons will pull along the block immediately in front of them. obsidian, bedrock, tile entities, and extended pistons cannot be pulled, just as they cannot be pushed.

Can you break obsidian with Pistons?

Obsidian cannot be pushed by pistons. It also cannot be pushed nor pulled by sticky pistons.

What can Pistons not push?

Pistons cannot push blocks into the void or beyond the top of the map. They also cannot push more than 12 blocks. If the requirements for a block to be pushed are not met, the piston simply does not extend.