Risks to consider in general are (also of the top of my head don't take as exhaustive):

* Electrocution

* Burning yourself

* Setting stuff on fire

* Fumes, both from soldering and overheating/burning plastic

There's a kind of balance between habits and awareness. Rely too much on awareness and you will miss some safety issue during a complicated repair. You need good habits, but rely too much on them and you won't notice when you finally make a mistake.

Those apply during repair processes. It's also possible to render a device unsafe to use, for example by damaging a li-ion battery or by a 'bodge' repair that circumvents a safety feature.