I can see them being a problem in towns and I can see them getting themselves into trouble on country lanes if they're used for farm access. On most roads, I'm not sure what effect the extra 2m will have in terms of safety – I find that in general, the bigger the lorry, the more careful the driver is.
But I do think the environmental impact will be bad. There will be the immediate impact from erosion of roadsides through wider turning circle and, some time later, from higher weights – because that's bound to happen. And there will be further inroads into freight on rail.
If all goes to plan, I shall be meeting a cycling HGV driver this weekend, so I'll ask his opinion.