So since club aircraft (well at least my club anyway) don't have an autopilot (well, they do, but IIRC they ALL are marked u/s) what's your opinion on a young, approx 170hr PPL/IMC handflying a 3-hour flight with about 75% or so IMC? (hypothetical situation of course! 
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It's OK but if you really did spend 3hrs in IMC you will end up really knackered for the time you don't want to be - the landing. This would be true for the most seasoned professional pilot, too. Personally I wouldn't do it.
Mind you, not many spamcans can stay up for 3hrs with a meaningful reserve...
On a more general note, there is obvious confusion here (from comments about FM Immunity etc) between
"UK style IFR" (which is basically flying around in Class G, often without any meaningful ATC service, popping in and out of cloud as necessary, and without any enroute clearance) and
IFR/airways flight (which is cleared enroute, under ATC direction i.e. Radar Control Service the whole way, on Eurocontrol routings whose MEAs would usually place you into freezing levels so the strategy is to head for VMC on top ASAP).
I don't think this "3hrs IMC" poster is doing the latter! In Europe, if you spend 3hrs in the airways in IMC (say FL120) you will likely end up carrying more ice than the father xmas sleigh in Spitzbergen.
He's clearly talking about doing this in UK Class G, and as I said it's OK but it's going to be helluva hard work. Also, in the winter, the flight is likely to be below 0C temperature so if really in IMC icing will be an issue, and flying at the low levels which one tends to in the UK (say 2000-3000ft, remaining below CAS) one has limited options for a descent into warmer air.
For me, a duff autopilot is a no-go item for IFR/airways. The workload is normally low, and I have had AP failures where I had to fly manually, but on occassions the workload can shoot up and then you need all the help you can get. I once had an AP failure on the way to Greece and had to fly by hand there and all the way back, but that was at high level (FL100+) and mostly VMC. But then I have a well equipped very stable plane.
Re nav, a decent GPS is a must, backed up by VOR/DME. But I wouldn't want to trust the crap avionics in the average crap rental spamcan which is what I used to fly in. I've had VORs which idented and indicated OK but were way off, and DMEs which idented, showed a reasonable figure but it was miles off. IMC training is done mostly in VMC (under the hood) so the stuff doesn't have to actually work. This is a generalisation but is something to watch.