I went from 10 years 737, including 8 as Capt, into RHS 747. The training was top-notch and I found it enjoyable to learn about the new considerations required for LH flying. Not so much wide-body but long-haul was the main difference. (Having said that, I guess widebody differences include the greater number of passengers and the attendant higher occurrence of medical issues.)
Although in many ways the 744 was not so different to the 737, it was a new airline, new SOPs, new routes and working with three or four flight-deck crew and up to 18 CC that took some getting used to. And as previously mentioned, you only get to fly three or four times a month and do a landing one or two times a month. Any individual destination may only come up once a year. With a large fleet, most times you are flying with strangers. So, each flight is a bit more of an ‘adventure’ and it just takes longer to get the hang of it. That’s what I found anyway.
I wouldn’t have been comfortable about going into the left seat for at least a couple of winter seasons. Little did I realise it would take 18 years! Now LHS on 787 after a brief spell on European A320 Ops.
As others have said, it’s not at all difficult but airlines want to minimise the training risk and so previous LH / WB experience is considered to be valuable.