To preface, the Plains of Eidolon bounty stage bonuses are exactly what they say, a bonus. If the topics discussed below do not get addressed, the world will not suddenly end.
There are, however, the following 2-3 specific stages are irritants to myself and players I have talked to, and could stand to be polished off a bit.
1. Liberation: Liberation as a fight against the inner workings of the game
Liberation has the bonus challenge of keeping the control percentage above 50% for the duration of the liberation timer.
Control decays while enemies are within the radius of the control zone.
Control decays faster the more enemies are within the zone.
The size of the zone is not determined by the number of players in the squad, and is larger than can be monitored by one player.
Enemies on dropships count as being within the zone, even before they land (potentially even before they reach the zone).
Dropship pathing has improved, but is still not perfect. Dropships can rarely still get stuck.
This is a short list of factors that in chorus produce the result of a solo player or small group struggling to complete the bonus challenge. The working strategy seems to be "Stand exactly where enemies drop and kill them as they touch the ground or, even better, take out the Firbolgs before they reach the zone." Liberation winds up feeling like you are fighting the underlying structure of the game, and not the enemies. Verdict: This challenge is not impossible, but the difficulty is a couple steps above all of other challenges. In addition, the difficulty of the challenge does not stem from the challenge itself, but from the challenge amplifying an underlying imbalance with the bounty stage. Potential solutions: Vary the size of the zone or control decay according to the number of players in a squad, or at least remove control decay from enemies still on dropships?
2. Capture: Capture as a race against your squad
Capture has the bonus challenge of capturing the target without killing any enemies.
I can keep this one brief; this bonus is trivial, but also trivial for a player to sabotage for their squad intentionally, or unintentionally. I have also heard reports that a latcher exploding counts against the challenge as a kill. Verdict: This challenge is easy for a prepared squad; however, this challenge does not bring any ease of mind, as you constantly anticipate a troll/unknowing player. Potential solutions: Increase the threshold for failure from just one kill, only count kills within the active zone, or at the very least ensure that latchers exploding do not count as kills? Honestly, this one does not seem to have immediately obvious, thorough solutions while maintaining the same challenge.
3. Assassination: "Kill what now?"
Assassination has the bonus challenge of drawing out the assassination target within one minute.
This one is also pretty easy to discuss; the struggle is not killing the enemies, it is finding the enemies to kill. In some cases, the number of enemies necessary for the challenge do not exist within the active zone within the time of the challenge. A full squad rarely has trouble with this challenge as squad members can divide focus, and even find and kill enemies outside the target zone. Verdict: This challenge is easy for a full group, but can be difficult to nearly impossible for a solo player or a small group, depending on spawns. Potential solutions: Increase the the challenge time by up to 30 seconds, or just ensure that there are a few more enemies within the zone at the start?