The main thing an aspirant has to keep in mind while preparing for a competitive exam without coaching is that there are thousands of aspirants who are burning the midnight oil to crack the same exam that you are going to write.
Often, when we prepare alone, we think that the effort that we put in is enough to sail you through the entrance. We would have definitely improved and would feel contented because we would have increased our knowledge from the day before. However, another person who might be far away would have not only internalised the same concept but also solved question papers pertaining to that topic. A person, studying at the comfort of their home, risks discounting this fact, and paying the price later.
Other than this, there is also the chance that you might lack a general direction on what topics are important and what topics are not. You have to find a way to correct this too. For example, in engineering entrance, Quadratic equations might not be as important as complex numbers or vice versa. A coaching institute does a lot of the research for you and you can better utilise your time in just studying.
However, given the way in which internet has made information ubiquitous, I don’t think that any of these factors are a major impediment these days.