For unimplemented stuff / placeholders I highly recommend exceptions instead of TODOs in any form (if suitable and your language of choice supports that). E.g. in Python:
But why only one or the other? The TODO is for your IDE to tap into so you can quickly and easily find outstanding work before the code is ready to be submitted as MR/PR (most IDE and even many simpler code editors have TODO/FIXME listing built in).
The exception is for runtime behaviour until the TODO is resolved: it's something you do mostly for yourself, not others. By the time others see your code, your TODOs have either been addressed, or they're acceptance criteria for follow-up work in your project tracker.
And that brings us to the third part that we _also_ need: the issue that the work the TODO talks about is for. Either as a checkbox task in a larger piece of work, or if it's big enough, taking up the entire issue. (because good project management means knowing what tasks are required/which work is outstanding, without opening a single file)
