If you have sand for soil, unless you live in a desert, it's very possibly (but not definitely) acidic and poor in nutrients. Desert soils are often sandy and alkaline, with caliche underneath. Although Miracle Gro can add needed fertilizer and minerals, it should be noted that some nitrogen fertilizers may contribute to soil acidification (acidification can make your soil even more sandy, so I've read). I don't know if your kind of Miracle Gro contributes to soil acidification, but fertilizers generally are sometimes stereotyped as leading to soil acidification over time (although only some of them do that); if soil gets too acidic, that could cause problems (see further below). This may be why some might think it could be bad to use for your purposes.
Fortunately, if your soil is too acidic, you can always add something alkaline to it, like lime, wood ash, rockdust, etc. which should help to level out the pH. Things high in calcium tend to be alkaline. You should not assume that your soil is acidic, however. Many people like to think you should always get a soil test in such situations, so as to avoid the possibility of making your soil more problematic than it was to start with (and to avoid unnecessary labor/time/costs due to error), and so you can add exactly what you need and nothing else. I'm not going to say you should always do that (since I kind of like to wing it sometimes, and learn what works for my soil by experimentation and intuition—and it's often less expensive), but there are enough people who will disagree with my methodology that maybe you would be one of them if you knew more about the situation. So, I recommend a soil test if you can get one.
If you can't get a soil test, don't give up hope, but so as not to raise ire from other-minded people, I'm not going to go on about how to deal with that situation (although it is possible). With traditional methods, it takes a lot less time to raise soil pH than to lower it (so, be very careful any time you decide to try to raise the soil pH—or any time you use something with high levels of calcium in it, like lime, wood ash, rockdust, etc.)
Having a soil pH that is too acidic or too alkaline can cause nutrient deficiencies and toxicities (due to nutrients being either too available or not available enough—whether or not the nutrients are there). This can result in stunted plants, stippled foliage, yellow foliage, and other stuff.
There are many other reasons why some people might think that adding Miracle Gro is a bad idea (and no, I'm not saying it's a bad idea by listing the reasons people sometimes have; I'm seeking to help you understand why people say what they say; I'm not trying to make you anti-Miracle Gro). I'll list some reasons that I've heard.
- It's not approved for organic gardening. There are lots of organic gardeners out there. It contains synthetic fertilizer salts.
- There are a lot of people who are against using phosphorus, whether because they think the soil always already has enough (whether or not it's available), or whether they think the phosphorus industry is unethical or otherwise harmful. Miracle Gro tends to contain appreciable levels of phosphorus (but not high levels compared to other fertilizers). Another reason people are against water-soluble phosphorus is that it can leach into the ground water. Some may assume that all phosphorus is water soluble, but this is not true. However, Miracle Gro tends to be entirely water soluble.
- It's quick-release. Some people don't believe in using quick-release fertilizers for some reason—probably because it doesn't happen in nature too much). I can appreciate this rationale at least where it pertains to acclimatization (where people are trying to breed plants that are not reliant on such fertilizers, as Joseph Lofthouse does with landraces—he raises plants without fertilizer and *cides—not particularly just quick-release).
- Using too much Miracle Gro may result in 'salty' soil. Salty doesn't refer specifically to sodium, by the way. A salt is a mineral or element that is bound to an acid. For instance, magnesium malate is a salt that people use for supplementing magnesium in humans (malate comes from malic acid, and magnesium is the mineral). Some people worry a lot about using too many fertilizer salts (since if soil is too salty, it is said to cause issues for plants).
- It's easy to burn plants with Miracle Gro. Some people don't like to risk burning plants. But, if you follow the directions, you shouldn't normally burn any plants (although it is possible). Be sure to research the salt index. Different fertilizer salts have different indexes. The higher the index, the greater the likelihood of plants being burned. I've personally found it easy to burn indoor pre-transplant vegetables with 24-8-16 Miracle Gro, but I don't think I've ever burned an outdoor plant or a houseplant with it (it works great for houseplants, in my experience). They make kinds that are more suited for vegetables, and I've used one for tomatoes (but it's not my favorite for pre-transplant tomatoes; I haven't tried it outdoors).
- I don't know all of the Miracle Gro products do, but some fertilizers kill some soil microbes, and a lot of people probably assume that Miracle Gro does by default. I know calcium nitrate, and potassium chloride, are said to kill soil microbes. I don't know about other fertilizer salts (other than at least some nitrates), however, and I don't know if Miracle Gro contains nitrates or potassium chloride (but it's a good possibility). Edit: My 24-8-16 All Purpose Plant Food (years old) contains potassium chloride, but no nitrates. Some people think urea kills soil life, but I haven't been able to verify whether this is true or false; it would seem to stimulate it if anything, from what I know about it.
- Some may think it's unhealthy. This may in some cases be because of reactions they or others have to certain fertilizer ingredients. Or, they may worry about fluoride, plthalates, lead, and other stuff, whether or not they're in Miracle Gro (they are in some fertilizers).
Also, realize that desert soils may be high in water soluble salts already (although in a 40-year-old garden I'm more doubtful of that); so, if you do have alkaline, sandy soil, you'd potentially have to worry about the salts in your fertilizer, even if the acidification isn't a concern. You'd probably want a soil test to find what, if anything, you need.