Is self-levelling a floor actually as easy as YouTube makes it look?

Is self-levelling compound actually a DIY job, or is it a trap? It looks so satisfying on YouTube, but I’m terrified of ending up with a floor that looks like the rolling hills of the Cotswolds. I’ve heard horror stories about it setting too fast or leaking through the floorboards into the room below. Has anyone here actually tackled this themselves, or is it worth the couple of hundred quid to just hire a pro? I’d love to know if it’s manageable for a novice or if I’m about to make a very permanent mistake.