My nephew built it one weekend when I took his blind mm away on a cruise. That is relevant, as it meant she couldn't get annoyed with his dark muttering and wouldn't accidentally clear up any parts as she didn't know what they were.
I don't think it was actually that tricky, but the sheer size adds to the complexity IYKWIM. For example, connecting the different sections without breaking another bit off as you do so.
He stores that one and the Flying Scotsman in special Lego display cases. Have you factored in how much space it will take up as you build it? It's hard to clear it away for dinner.