How can Chum fail?
- Chum's subroutine can be broken
- the underlying ice's subroutines can be broken
- the runner can jack out after encountering Chum (we'll call this a draw)
Chum works well in tandem with cards like Hunter, Snowflake, and Checkpoint. That is to say: cards that have high strength/cost ratio, because their subroutines are flaky or weak. This creates a taxing ice combo.
The great tragedy of Chum comes when you open your initial hand to find it flying solo, unaccompanied with underlying ice. This leaves you to install it as a bluff - probably permanently stranding it. Alternatively, you can try to dig for better ice, but the Chum has taken that card's slot in your deck. You could have Yagura or Quandary in play now!
Chum is also deceptively expensive. You always want it's install cost to be at least 1 credit, and you're also anteing time to install the underlying ice.
Ironically, Chum does work pretty well in front of ... more Chum! It's pretty high strength. Same goes for Inazuma, Sensei, and so on down the line of positional code gate ice. Just beware Gordian Blade - it can slice through the thickest knots of data.
Perhaps Chum works best in decks swimming in ice - NEXT Design comes to mind.
Chum is also strong in Blue Sun - it covers Weyland's natural dearth of code gates. If it's installed as a bluff as the innermost layer, it can be bounced back after the bluff is called. Executive Boot Camp is perfect for this.