My concern would be how bad the drive failure is since standard macOS apps (even data recovery apps) and even macOS itself are not equipped to handle the errors produced by a failing drive. Data recovery apps are designed to recover data from healthy drive where data has been accidentally deleted or the file system corrupted where it won't mount. When an app tries to access a damaged portion of the drive, the app & OS will get stuck trying to deal with the error which many times will make the drive failure worse to the point where even an expensive professional data recovery service will be unable to retrieve the data.
If you try to modify the drive, then you risk losing the data. First rule of data recovery is not to modify the source drive in any way and if the drive is failing, make a low level bit for bit clone before the errors get so bad even this is impossible. I know of only one command line Linux utility which is able to produce such a clone as the utility can skip the bad sections of the drive, then on another pass attempt to access the damaged sections. Even then, the clone of a broken Fusion Drive will still likely not mount, but at least any attempts to use a data recovery app won't increase the damage to the drive and risk the data. Ideally, another clone of the clone should be made if any modifications to the clone drive are to be attempted in case a mistake is made.
In fact I always check the health of the hard drive in order to determine how to proceed since the worse the failure, the more careful you must be and the less likely data can be recovered by amateur means.
Most people on these forums will not know whether it is possible to mount the hard drive portion of the Fusion Drive. I have rarely worked with Fusion Drives at all, although I do have an understanding of some partition & file system details, but even with my knowledge I cannot say if it is possible to mount just the hard drive portion of a Fusion Drive which has a good hard drive, much less a drive which has a hardware failure (less likely it can be mounted with a hardware failure especially when the drive should not be modified if data recovery is to be performed).
You usually only get one chance at recovering data from a failing drive, so make sure you choose wisely with how you proceed. A professional data recovery service such as Drive Savers is able to do a better job as they have special expensive equipment & software to use if necessary, plus they tend to swap hardware components as well if needed.
