I take major issue with that because it's a definition for real life when difficulty matters/exists. In a game though difficulty is supplemented for time (how quickly it takes) and cost (how cheaply it can be done). Often the player has to try and find a balance between these 2 things which then produces that players 'game difficulty' in achieving the quest.
In addition to this I would argue that if a quest is meant to be hard or difficult to do then why are there a number of quest that can be done for little cost and 2 clicks of a button? Not very difficult or challenging so either way BB fails.
The guild quest system is a complete mess by any definition. It's either too quick and cheap, too expensive, too time consuming or in some cases counterproductive. It is the real world equivalent of taking 2 steps back in order to go 1 step forward rather than 1 step back and 2 steps forward.
In just about every game made before f2p came a long a player would take up a quest with the understanding they would get something out of it at the end even if that reward was the ability to progress to the next level or quest in the chain in order to get a special sword or chain mail +1.
That is the definition of a quest when applied to gaming and is what most players would expect. You do something 'hard' or laborious and in exchange it helps you, progresses you or benefits you in some way without making you worse off then you previously were before you undertook the quest. Otherwise why the heck would the average player bother with it knowing it would cost them more then the reward is worth?
If it was about comradeship and doing something together it wouldn't be implemented in such a pathetic and ludicrous way while called a guild quest.
Guild quests are no different to single player quests. Do an adventure, spend some resources etc... The only change comes in the form of them accumulating till enough people reach a magical number wish outputs a few more extra guild coins. This is nowhere close to what I'd call a guild quest, more like a single player guild annoyance in most cases.
A guild quest should be something like a 20 player adventure where guild members need to fight their way through completely random enemy camps and reach the castle, dragons lair, etc to get the well worth it reward.
Something where guild members actually interact and meat on the battle field, strategise, plan and work together doing something rather than just sending off more resources into a sink hole void never to be seen again or completing an adventure by yourself again and again.