So first lets look at how big an ability this really is
Most AI cities toward the end of the medieval era tend to have between 40-60% culture defense.
Assuming a castle,
For your maces/trebs/knights etc this is a flat 100% regardless of culture level.
However for muskets ignoring walls shows itself in a way similar to an attack bonus
So for a city with 60% culture, you gain +40% attack, for a city with 40% culture, the bonus is 60%, and so on for other culture levels and wall/castle setups.
On the face of it at least, this bonus is pretty big. The downside however, is that these defenses can be bombarded away, although some things happen here that IMO will make a big difference to the true value of muskets.
-Firstly walls and castles reduce bombardment damage done to the city by a total of 75%, this means that sieges will take an extremely long time to break the defences especially as a treb with accuracy will only cause a mere 6% reduction per turn!
-Secondly, the bombardment damage is split between culutre, and the walls based on the ratio of the two.
i.e. if a city has only 20% culture, and 100% from walls and castle, 4/5 of the defense reduction will be absorbed by the walls/castle.
If the culture defense value is 80% then the trebuchet does a lot better with only 1/5 being absorbed by the walls/castle
Regardless of the cultural defense, both cities will take the same number of trebuchet turns to reduce to 0.
As muskets ignore the wall defense this stuff means that, at lower cultural defenses a good percentage of the trebs bombardment is completely wasted, so its probably better to just charge in while, at higher cultures bombardment will help more.
The waste still exists though, I don't know how much of an issue this causes to musketmen as a whole.
Bizzarely this also applies to cannons (probably artillery too), against cities with walls and castles they are forced to reduce the defense from 100%, just as any other siege unit. Although in this case the 75% defense against bombardment is removed.
*As a side note, the trebs higher bombardment ability than cannons (they're equal to artillery!) combined with walls being obsolete at rifling may make some accuracy promoted ones worth keeping post steel.
Now the other things
Being draftable does help offset the lack of any units that can upgrade into muskets a bit, and I feel that the ignoring walls bonus as it needs no promotions, works quite well with the draft.
The other main advantage for muskets, is they are pretty hard to counter.
On the flat knights will beat them yes, but muskets can gain defensive bonuses, and can access the formation promotion (easily if AGG, PRO or CHA) while the anti musket promotion Pinch, is only available with gunpowder. To me this says muskets are potentially a major breakthrough if you reach gunpowder before your oppponent.
However, without the CR line of promotions, they are signifacntly worse at taking cities without castles than maces are.