Many of our mnemonic systems rely on making visual associations. Visual associations are very effective in recalling concrete material that can be easily pictured in the mind. Abstract material is often difficult to visualize, although the phonetic system does do a good job at making numbers concrete.
For abstract material, verbal associations are much more effective. Verbal associations are things like rhymes, acronyms and acrostics.
You should never rely on one mnemonic system for all of your learning. When you are studying something, try to determine if the material is abstract or concrete and pick a mnemonic system that best suits the material.