My friend and I used to use a cactus in a hole as a trash can for unwanted garbage caught while fishing (the massive amounts of unneeded fish, saddles after a full double chest, leather boots, etc) and I had to change it to a lava source block because the cactus wasn't breaking the stuff anymore.
This report does not encompass Legacy Console Minecraft versions, as this is a Bedrock Minecraft report. If you're experiencing issues, please create a report for it here.
Confirmed on Samsung Galaxy S6 Android 5.1.1 MCPE 0.14.2.
Mobs placed on top of cactus blocks also do not take damage.