There are 3 difficulties with nuclear fusion: 1. temperature, 2. time and 3. containment.
1. The temperature for starting the fusion reaction is around 40 million Kelvin. This extreme temperature is very hard to accomplish.
2. Time for the nuclei to fuse is 1 second. The problem is that nuclei have to be held close enough for long enough at 40 million Kelvin.
3. Containment is the major issue because of the extremely high temperature involved. At 40 million Kelvin everything becomes a gas. In other words, it vaporizes. So the nuclei has to be contained via magnetic field in a device called the magnetic bottle. If the bottle leeks, no fusion reaction occurs.
Safety is also an issue when talking about containment because the extremely high temperature.