High tackles are illegal because it puts the person being tackled in danger. Serious injuries to the neck can be sustained this way, including whiplash, cartilage injury in the throat and even paralysis. It can lead to career limiting injuries.
When opposition players pull the lock down by knocking his feet out from under him, the player can land on his back or neck, causing serious injury to the neck or back. Other common injuries sustained this way include fractures to the arm and dislocation of the shoulder.
Scrums usually collapse if the prop or hooker lowers his shoulder or tries to pull other players down. If the scrum collapses, any of the front row players can sustain serious neck injuries. Scrum collapses can be prevented by letting players engage in the scrum at the same time.
During rucks and mauls, players often stamp on other people. One can imagine that when a man weighing 120 kg wearing studded rugby boots stands on someone else, serious injuries can be sustained.
Being tackled while fielding a high ball:
Injuries can be sustained when a player jumps up into the air to catch a high ball and an opposition player tackles the player while he is in the air. If the player lands on his neck or back, serious injuries can be sustained.
Late tackles are dangerous because the player is caught unaware and is not prepared to be tackled.
A shoulder charge is usually done with malicious intent with the sole purpose of taking the other player out and can cause unnecessary injuries.
The same injuries can be sustained as during a ruck or maul.