Moderately difficult. Certainly more than fibreglass probably a bit less than timber.
Initial hull preparation of a steel hull is most important. Point One.
Most irksome thing about steel is that everything weeps just a little and in a remarkably short period of time you end up with rust streaks on the hull. These are not harmful just unsightly. A quick rub down with a mild acid solution (mea culpa I've forgotten the type of acid, boracic ????) and its all gone but leave her on the mooring untouched for a week or so and it will be back. You can minimise this weeping but it comes from under deck fittings, such as stanchion attachment points, mainly. I suspect the only way to avoid this would be to have everything welded to the hull and deck, nothing bolted.
On deck you also need to watch for any cracks in the paint. Left untended they can develop into real problems as the moisture will migrate under the paint surface until you will find a sheet of paint will simply come away revealing an ugly patch of rust. Again not all that difficult to fix with the aid of a Dremel or similar and products like POR, but you do need to keep on top of it.
They say that steel boats rust from the inside out, I'm not convinced about this.
Inside the main thing is to ensure that all surfaces are intitially perfect then keep everything dry dry dry. Before you leave the boat, mop out any moisture in the bilges and don't allow moisture to accumulate for any period of time if you are staying on board. Make sure all limber holes are kept clear of any gunk at all times. Also you must be able to access all, and I mean all, internal areas of the hull. If any parts of the hull have been lined with carpet , get rid of the damn stuff.
Probably sounds worse than it really is.
Obviously the best thing about steel is its strength when presented with things like floating trees trunks, other boats, half submerged containers and the odd reef but I'd still not like to put any of these to the test.
On the other hand it is fair to say that in a position of disputed right of way with something light and plastic and a well placed " no problem, we're steel" serves to clarify the situation.
Our current boat is steel and while we are in the middle of buying fibreglass that is not from any disatisfaction with steel, we are simply going a bit larger in size and the grp boat seemed a practical choice for us. Had a suitable steel boat come along I would have been tempted but hull survey would need to have come up with a perfect report card. A poorly built steel hull would be second only to ferro as a hull to be avoided.
My final thought is that for liveaboard cruising steel is second only to aluminium as a choice of material but for a day sailor or weekend cruiser it's probably more trouble than it is worth. Final point is that steel really does not come into it's own until around 40'. Much under than the inherent weight of the material itself results in less than exhilarating performance in light airs. As an example our 34'er is painfully slow in anything under five knots and really doesn't start to kick up her heels until it tops ten.