Is it ok for wood to be near a heater? Or any decorations at all? Will it create a hot spot and crack and the heater or glass of the tank?
A heater inside an aquarium is going to be relatively safe as long as it is rated for aquarium use as sold in pet or fish shops. A general rule of thumb is cheaper products vs. industry average will be more susceptible to breaking, cracking (glass heaters), and the thermostat becoming faulty (either staying on and making the tank too warm, or shutting off and the tank will cool down too fast). In my personal experience, if I know that solid object could potentially come in contact with the heater, I will use a titanium heater which are generally more expensive. Rocks in your sump, would in your display tank could possible flow or be pushed into the glass heater and crack it!
Aquarium heaters aren't going to become hot enough in water to crack glass or cause any real damage if that's also something you're worried about. It's recommended to not lay the heater flat on the tank bottom or substrate. And definitely keep the whole heater submerged! Otherwise the exposed part could become hot enough to melt through a sleeve or container that the heater is wrapped in.
As with all aquarium equipment, it's not only a best practice, but crucial to safety that you turn off all equipment before doing water changes, cleaning, putting your hands in the tank, etc.