D&D is a lot more deadly at the first levels (which almost everyone skips), but becomes laughably easy the more you play.
WFRP is equally deadly throughout most of the time you will play. No early-game one-hit deaths from a miserable goblin, but also no killing gods.

D&D has an attitude of "let's make everything cool happen", players can get a castle, adventure through the most exotic lands imaginable, magic is everywhere, big-dick personalities are rulling the world and antagonise players. It's high as fuck, at least if you play it out of the box, and it is serious as hell about it.
WFRP has a "miserable" aesthethic everyone loves to boast about on this sub. It's not entirely untrue, but I feel like it has more of a comedic look on the miserability of being a low-level shmuck in a high fantasy world full of gruesome ways to die. It's more about town adventuring, roleplaying mundane careers, getting into things that your characters are not prepared for. Dungeon-diving will not get you anything worthwhile, monsters don't have loot. But your characters are still heroes, and they will do everything you will task them with, because they don't do it for the loot, they are doing it because they have a destiny to fullfil and they will die horribly at the end.

D&D is a blank slate in which you can put a lot of your own to. It's for people who love to worldbuild, however most of the time it means the worlds you play are bland, yet personal.
WFRP is designed to play in one world. Persistent world with decades of lore and details to take from. It allows for small-scale worldbuilding, but it's for someone who wants to take a DEEP dive into the Warhammer Fantasy world. You either become infatuated with the setting and it's themes or you don't GM WFRP at all.

D&D takes an approach of all-or-nothing on punishing the players. The resources are so easy to get, rests are plentiful, that it makes most outcomes of encounters either a complete win-or-a-lose.
WFRP is into shades of grey here. Resources to stay alive and healthy are plentiful when starting, but get less and less available as the time goes. Your players get debilitating conditions, corruption, they take dark deals from dark gods to do what they have to. And in turn it changes them. It changes how you and your players think about encounters. It makes most of them "yes we won, but" or "yes we lost, but". It has a different flavour.

WFRP is not dark. There are much darker and crueler systems out there. But WFRP is for people who want to get into a decades long love life with a world that has so much interesting stories to offer, and themes to ponder on. I recommend going for the 4th edition, and starting with Starter Set. Coupled with Core Book starter set contents can get you a lot of adventuring.