Are there any issues with Power Macintosh computers using Composite or Non-Composite DIMMs?

Apple does not recommend or support composite DIMMs in any Macintosh model which uses 168-pin DIMMs.

Composite DIMMs, like composite SIMMs, are made up of lower-density components to construct a single bank of memory. Combining multiple chips to achieve a larger sum of memory can load down the signal to a point where the signal becomes unreliable.

Apple's Developer Note states the following:

The number of DRAM devices in a RAM DIMM for the Power Macintosh 9500 computers is constrained by the load limits of the unbuffered signals. On each DIMM, a maximum of two devices can be connected to each data line and a maximum of eight devices can be connected to each /RAS line.

DIMMs used with Apple's products are to meet the following JEDEC Joint Electron Device Engineering Council standards. Its mechanical design is defined by the MO-161 specification published by the JEDEC JC-11 committee; its electrical characteristics are defined by the JEDEC Standard No. 21-C.