Bow Mastery and some of the passives in the tree do not play nicely with channeled flurry (or any potential future channeled abilities that get added). Bow Mastery/Barrage of Pain and Arrow Storm only occur at the start of the channel leading to the user having to spam click 5 times at the start to build up stacks and then constantly pulse the ability every 4 seconds to maintain the stacks to achieve maximum dps. This causes your channeled ability to not feel like a channeled ability anymore. I have to constantly keep an eye on my buff bar to time my pulses of flurry so I don’t lose my stacks and I personally find this level of micromanaging really annoying and frankly defeats the purpose of using a channeled ability in the first place. As it plays now, I see no reason to play a channeled flurry + fusillade build over a straight multishot build since you either have to do annoying micromanaging culminating in you stopping your attack for no good reason or miss out on a really good passive bonus and multiple nodes in the tree that would be amazing otherwise and no other build has to deal with.
I suggest making Bow Mastery/Barrage of Pain like Accelerating Impact in the flurry skill tree. It should add/refresh stacks every x seconds if channeling an ability. You can play around with the stack rate if you think it will be too powerful, anything between 1-3 seconds would be a massive improvement compared to where it stands now.
Also Arrow Storm should automatically activate if it comes off cooldown while you are currently channeling.
Making these two small changes would make playing this setup feel so much better since it would actually feel like you are using a channeled ability that has multiple ways to ramp up over time. This makes it great to play in multiplayer where other people can distract your opponents to give you the safety you need to be a really good source of damage. Or in solo it makes it worthwhile to use abilities like decoy or turrets to create these openings to safely stand still and deal out your damage resulting in more interesting/engaging fights.
Also Ethereal Arrows seems to do literally nothing for channeled flurry. I haven’t actually tested damage numbers on enemies but the DPS listed on the ability mouse-over tooltip doesn’t change when adding/removing points into Ethereal Arrows. If it does do anything it probably does next to nothing anyways because it says it uses its mana cost which is 1, while its channel cost is 13 with fusilade. If it already does these things, then the tooltips are seriously lacking important information and need to be updated. Otherwise it would be nice if it worked using the channel cost for channeled abilities even if its has separate damage scaling (50% per point for single cast, x% per point for channels). It also doesn’t help that Channeled Flurry is often paired with Fusillade which makes the Master Archer passive adding arrows to multishot very appealing, yet you are forced to go through Ethereal Arrows that seemingly does nothing for you so the synergy here feels lacking. Change or no change, you should explain how it interacts with channeled abilities in the tooltip (probably in the hold alt for more info section) because it is important information and not obvious how the two things interact.
Sorcerer’s passive Arcane Momentum also has the issue of not generating stacks during a channel, could be others as well.