Is there any valid reason to not choose the C-RAW option on recent Canon cameras?

That's kind of like asking, "Is there any valid reason to not choose the JPEG option on recent digital cameras?"

Of course the answer to that is, "Not unless you need the complete raw data."

Likewise, the answer to your question is, "Not unless you find yourself in a situation where the complete raw data will allow you a better result in post processing than a lossy compressed C-RAW file will."

Granted, the edge cases where this will be the case will be far more extreme than the number of cases where a losslessly compressed raw file will be superior to a jpeg file for further editing.

Of course, the biggest thing preventing an easy answer to this question is that you usually do not know until after the fact exactly when you need every scrap of information contained in a lossless raw file. And therein lies the rub: by the time you know whether you need a lossless raw file or whether a minimally lossy C-RAW file will do, you're already past the point at which your decision has already been made.

So how do I decide before I shoot?

Experiment with some non-critical shooting situations that represent the kinds of things you shoot that require the most extreme adjustments in your raw editing workflow. Situations where you know that when you capture the shot you're going to need to push the shadows significantly in order to expose low enough to not blow highlights as you capture the scene. Shoot scenes with C-RAW, then shoot the exact same shots with the exact same exposure settings using the lossless compression option.

Then see how much difference it makes when you get on the computer to do your raw development. Do you:

• Get near identical results that are visually indistinguishable from each other?
• Have to do more work to get to that point with the C-RAW file? Or do the same adjustment steps get near identical results? If more work is required, is the extra time worth the storage cost savings and decreased buffer depth when shooting?
• If there is an observable difference, is that difference worth the burst rate penalty for what you're shooting? Or is the increased buffer capacity when shooting worth more to you?
• If there is an observable difference, is that difference worth the storage cost penalty for what you're shooting? Or is the reduced per-shot storage capacity needed worth more to you?

These are individual judgements that only you can make. The answers may well be different for you than for me or anyone else.

Other things to consider:

• Do you often find yourself needing to radically correct for missed exposure or rapidly changing lighting conditions in fast moving shooting environments?
• Do you find yourself shooting in marginal light that will require a lot of color correction in post? Or do you usually shoot in fuller spectrum light, either natural or that you provide?
• Do you usually shoot static scenes that allow more preparation, careful exposure metering and adjustment, etc?
• Do you shoot hundreds or even thousands of frames per shoot? Or only a handful?
• Are you able to pass the increased storage costs of larger files to your clients? Or are your shots only for your own personal usage?
• Is the shooting situation a "mission critical" one? Wedding? Graduation ceremony? Other event that must be captured effectively in one take?

You might decide that some situations would benefit more from the increased security of having the complete raw data at your disposal, or you might find that it never makes a difference for what you shoot.