Problems with mnemonics

1. In the long term, relying on mnemonics is usually less effective than making  information meaningful.
2. If mnemonics helps you by (1) forcing you to pay attention to the information (usually by having you turn the information into an image) and (2) giving you retrieval cues to trigger recall of that information that you wouldn't normally have (e.g., a peg word if you are using the peg-word system, a place if you are using the method of loci), you can do those two things without mnemonics: Just come up with your own examples.
3. Using mnemonics usually means doing at least two things: (1) encoding the information by changing hard-to-remember information into an easier-to-remember code word or image (e.g., converting "Huron, Ontario, Michigan, Erie, and Superior" into "HOMES" or converting "sensory register" into a picture of a cash register with an eye) and then (2) retrieving the information by decoding the code word or image back into the hard-to-remember information. Unfortunately, you will probably not be able to decode the word or image unless you have already stored the key information (e.g., if you don't know the names of the Great Lakes, remembering "HOMES" doesn't help you; if you don't know what a sensory register is, recalling an image of a cash register having an eye might serve only to freak you out). So, relying exclusively on mnemonics to remember abstract information is probably a bad idea.
4. Just as many people don't take the time and effort to set up a system of labeled pegs and shelves needed to organized their garage, many people don't take the time and effort to set up a mnemonic system. In addition, people are also discouraged because, in addition to the time and energy need to set up a mnemonic system, it also takes time and energy to come up with vivid, distinctive visual images usually needed to use a mnemonic system once it is set up. 
5. Although you can keep using the same system over and over, doing so runs the risk of having interference problems. For example, the first time you use a peg word system, there is only one thing that has ever been associated with your first peg word. So, when you think of that peg word, you easily retrieve that one thing. However, suppose you use that peg word system 20 more times. Now, instead of retrieving only the last thing you connected to that peg word, you may retrieve one one or more of the 20 other things you had linked to that peg word.
6. Using mnemonics usually means creating an episodic memory of events. For example, if you used the method of loci to memorize concepts, your memory is sort of a video of an episode in which you take a particular mental walk. To understand why such an episodic memory would have two disadvantages over forming a semantic memory, think of an episodic memory as being like a youTube video whereas a semantic memory is more like a transcript of that video's information that has been edited into a table--a table that contains hyperlinks both to and from relevant examples and concepts.
   1. To get to the course information in the episodic memory created by mnemonics, you will probably have to replay some or almost all of the recorded episode. For example, if you used the method of loci and the information was stored in the middle of your walk, you might have to go half way through your walk to get to the information. (If you have ever seen someone who has memorized a lot of information and can recite it perfectly, but, if interrupted, must start all over at the beginning, you may have seen a limitation of such a memory strategy.) If, on the other hand, you had formed a semantic memory of the information, finding the information would be much quicker. Rather than being like having to watch a video from the beginning until you heard the concept's name, it would be more like doing a computer search for that concept's name in a transcript of that video.
   2. To retrieve the words you need from your mnemonics-created episodic memory, you must retrieve unnecessary "video" (e.g., the images used to encode the information) and unnecessary context such as where you were when you learned the information (e.g., in the method of loci, in addition to the concept, you are retrieving the place in the mental walk where you stored the information). In contrast, to retrieve information from your semantic memory (your transcript/table of the information), you  retrieve just the words that contain the information you need: You don't recall irrelevant context such as where you were when you learned the information.  If, in retrieving a semantic memory, you do retrieve more than the concept, you are probably retrieving relevant "hyperlinked" examples and concepts.