I don't know about the Spanish wildfires, but I know in America there's a whole slew of challenges associated.

The first is the unpredictability of wildfires. You don't want to over-evacuate, that causes unnecessary disruption. But conditions can change rapidly, and a plan that looked good 6 hours ago can quickly become bunk.

Another is speed. Wildfires can travel much, much faster than people's intuition leads them to think. So, it can go from "over there" to "oh shit" quick enough to be life threatening. Wildfires counter-intuitively go uphill faster than downhill, winds can pick up, blowing embers can cause it to jump, etc.

A third is that some people try to protect their property. This is actually possible if you've taken proper precautions of removing burnables from your property and have a garden hose that you are 100% certain will keep running. This is not recommended, but some people will try and that is their right. It's a calculated risk they have to decide for themselves to take or not.

Fourth might be various other failures to evacuate. Evacuation order didn't arrive in time, elderly resident didn't hear in time/can't move fast enough, someone got stuck in evacuation traffic, mental illness, stubborn conspiracy nut thinks tin foil hats protect against wildfire, who knows, it'll vary case-to-case.

Fifth could be firefighter deaths. It's a highly dangerous job with a significant fatality rate, and our wildlands firefighters (colloquially called "hotshots") are basically like the special forces of firefighting. They airdrop into wilderness areas with whatever gear they can carry, with instructions to change the course of an oncoming force of nature. They have the tools (chainsaws and shovels mostly) and training to do this, but it's still exactly as dangerous as it sounds.

Wildlands firefighting is really fascinating stuff. I got an audiobook ages ago written by a retired one that talked a lot about the process. Was like 100 pages or something and super interesting, highly recommend.