Even given your definition of viable, I still think C3 Archer can keep up vs Musketeer, although maybe not in your build.
Going into c2->c3 you ‘can’ get:
5% more evasion bonus to your whole party.
more attack speed/attack range on kneeling shot
More damage on Multishot
twin arrows which I believe now have 6 overheat charges and 7s cd and +50% damage bonus against cloth
Critical Shot from Ranger does diminish the benefit of twin arrows somewhat, but the rest of the bonus are something worth considering.
There’s also things outside of just skills to consider.
Archer/Range/SR skills can be used with crossbow/pistol, meaning you can use Mana-Mana for insane bonus damage, as well as the crossbow mastery of ignoring 25% of target’s defense, both of which will not be used when using Musket skill because you’ll switch to a musket.
Another thing is the actual cast time of skills.
While Musketeer skills cast quite fast, it still take up some amount of time. If your other skills already took up all your time til now then the time you spend using Musketeer skills replaces those skills you used before, thus the actual ‘gain’ of adding Musketeer to your list might not be as big as number on paper provides. (Retreat shot takes up a fair bit of time)
Well, that’s my opinion on balancing topic anyway, there are other factors which are harder to quantify (like player’s own playing habit or whether the build is aiming to party more, solo, PvP, world boss farming, etc) so a build might not seem viable from one view point might be quite powerful from another.