With respect to answer which I got from a duty engineer in TWI the 1st is true.
The answer is as follow:
Table 8 of BS EN 287 indicates that welding a single-V butt weld without backing qualifies for welding any of the other butt weld joint configurations. From this, we can conclude that it is the most difficult.
Shane,
FYI we often use the term "asymmetrical" to describe a double-vee or double-bevel preparation that is something other than equal on both sides, for example one side is beveled to one-third of the thickness while the other side is beveled to two-thirds of the thickness. Of course, we also use butt joint and not butt weld.