What is tough about 18 month age gap

[Mindisboggled]

I've read a few threads about this age gap and lots of parents are saying it's tough but gets easier.
But can I can what did you find tough?
I'm getting organised for my dc2 arrival and with a dh who works shifta/away I will be tackling dinner/bedtime alone alot so want to be ready (as best as I can)

I have this age gap, DS is now 21 months and DD is 3 months. I can only say from personal experience but have found that DS was just a little bit too young to understand what was happening when his sister arrived and his language wasn’t as developed as it is now so he was often getting frustrated and this resulted in a couple of episodes where he threw things at DD. DH also works away so I do all bedtimes on my own, I just make sure DD has been fed before I bath DS and this usually buys me 20 or so. Sometimes I put DD in the bath with him or sometimes just put her in her mobile chair thing.
I found best thing to do is be consistent with older child and they soon learn boundaries; I never let DS interrupt DD when she’s feeding so have always just repeated ‘baby needs milk’ to him and now he knows when I say that he has to wait. It is hard but nowhere near as difficult as I imagined and you soon get used to an extra baby being around. DD has started smiling now and DS loves this and is now interacting with her much better.

Thanks for your reply. I'm fully expecting it to be hard going at the start but just curious to know what other think are the hard parts.
I cant wait for new baby to be here but admittedly a little nervous.