Is writing a fantasy novel generally more challenging than writing a romance novel?

Yes, for a number of reasons:

Fantasy novels are usually set in a different world, meaning you, the writer, must come up with plot, characters, and world. Romance (especially contemporary romance) is usually set in a world you don’t have to invent.

Classic romance follows a firmly established formula with firmly established story beats. You already have a template to follow. Fantasy doesn’t.

Romance is typically ~65,000 words. It’s one of the few genres publishers will consider if it’s less than 80,000 words. Fantasy is more typically 120,000–220,000 words.

Romance has a simpler narrative structure, and smaller cast: typically one POV character, the POV character’s romantic interest, the adversary character keeping them apart, a handful of supporting characters. Fantasy usually has a larger stable of characters, often including multiple POV characters, with more complex relationships.

Fantasy is usually thematically more complex.

That’s not even all. I could keep going, but you get the point. Writing fantasy is generally more challenging by far; it’s not even close.