There are negatives to consider. Firstly you need trees - you can use the system without trees, but it is designed as a hammock and works best when used as one.
True. If you are going places where there are no trees hammocks are not really a viable option. You can do it but it is not the right system.
It's more complicated to erect
Erecting a hammock is more complicated than rolling out a bivi bag
but it is not much more complicated than finding a level site and setting up your tarp. In a wooded environment the saving in time finding a pitch alone would make the hammock quicker. Even ignoring this once you are used to the system we are taking an extra minute or two of work all of which will be under the tarp so it should not be a problem.
it's not so convenient to get out for a pee or to make a midnight brew.
Now is where I really start to disagree with you. Using an underblanket, getting up in the night is much easier than a bivi bag. In the hammock you throw back the blanket or sleeping bag covering you, swing your legs out and wander of for a pee. You can put your boots on if you feel the need. Similarly if you want to lean out and set a brew going you can. You can pull yourself up into a more sitting position to drink your brew or swing your legs over the edge of the hammock and sit enjoying the sights and your brew.
Hammocks may seem unstable and difficult to move about in but the truth is they are very stable and you will not struggle to get in and out. Using a sleeping bag and a thermarest in them can be more difficult but still not bad.
You are also pretty much cocooned, some prefer greater visibility..
You still get pretty good viability from a hammock. More so from the group buy style than the HH. If you are lying at an angle in the hammock you are flatter and should not really be cocooned.
and they are not so good for use near a fire
Why? I have the fire in a firebox close enough that with a bit of a swing i can place wood into it. Certainly it is under the spread of my tarp so no farther than 1.5m from the centreline of the tarp. Plus in the hammock you are off the ground so get the heat of the fire better than being on the ground.
you need a bigger tarp because of the off-the-ground exposure.
I certainly think you need a longer tarp. 3 to 3,5m in length but Ray Mears uses a Hootchie with his. Seems too short to me but it can be done. I have to say I like bigger tarps though. I like a good sized living area when I am out and in Scotland rain is never far away so I need room to fit all my gear and be comfy.
You pretty much need an underblanket apart from in the hight of summer, because of wind chill cooling yer bum
A thermarest will do fine. With a thermarest and a hammock you are as warm as being on the ground and 10 times more comfy
. With an underblanket you are as warm and a 100 times more comfy
but you will get on fine with the thermarest in your sleeping bag. I have hammocked at -6 with a thermarest in my sleeping bag and was fine and warm.
it does seem like their might be a bulk and weight penalty with the system.
This really depends on what you want comfort wise. For a ground based bivi, mat, sleeping bag and tarp you just need to substitute the bivi bag with a hammock and tape to change system. Of course if you want more comfort you might want a bigger tarp and an underbalnket but if you want more comfort on the ground you will get a thicker full length mat and a bigger tarp as well.
I am not having a go at Martyn. Hammocks are certainly not for all or for all conditions but there are some misconceptions out there.
Easiest solution is for Martyn to come up here and I will take him for a paddle and an overnight in hammocks. that way not only will he never go to ground again but he will buy himself the canoe he needs to finish kitting out that Defender of his properly
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