Yes, anyone can make a will on a will form, online or on the back of an envelope and if it is properly witnessed it is legal BUT most disputes about will are caused by these 'simple' wills for simple estates.
Wording in a will has to be very precise and free from any doubts about exactly what is meant. All of us, all the time, say and write things that have an implicit back story that we know the person who hears or reads it will fully understand and add to their interpretation of our communication.
Once you are dead your will has to be read and the wording absolutely clear on the face of it and not dependent on your child, or DH/W, or friend telling someone what you really meant. There needs to be absolute certainty that you were fully in your right mind when you made it and not under any kind of coercion. If a solicitor has any doubts on any of these factors they can and will ask for a medical report and need to do so.
It is very easy to get on one's high horse when you are the will maker jingl, but most people feel different when they are at the receiving end of a will that they suspect was made by someone one withot capacity or under duress. This problem has arisen in several GN threads.