Yes, the loading sequence for a muzzle-loading musket would be more complex than the later loading with metallic cartridges. Failure to adhere to the sequence might result in the gun being temporarily disabled, or permanently damaged. Very specific instructions were given as to exactly how the musket was held, loaded, fired, etc. and soldiers were constantly drilled in it, as you can read here (on p. 91) . However, it would not put a serious strain on them: most soldiers of average intelligence and strength would learn the drill to the point of following the sequence automatically, just as people today learn and internalize many complex tasks to the point they can do them without thinking intensely- the very common task of driving a car, for example. Note that soldiers also had to work to march in formations, wheel and maneuver- handling a gun was only one of their memorized tasks. That need to drill soldiers to work uniformly as a team pre-dates firearms: infantry with spears also had to do some complex things, and had to drill to memorize them.

As has been noted here before, being trained to use a musket was easier than being trained to draw a longbow because not everyone was capable of drawing a longbow. That required not just drill but developing physical strength, and not everyone could do it because of that.

Yes, the earliest formulations of gunpowder were quite variable, and so the earliest use of it was in cannon firing at large targets ( like walls) and hand-cannon firing at large massed formations. In the 14th c., therefore, bowmen were still very useful. But as better formulations were worked out in the 15th. c., accurate small arms became possible and quickly began to replace bows.

if your are talking about a brown bess musket, it is not difficult to use.

the cartridges, which were paper, were kept in a cartridge box

https://3.bp.blogspot.com/-RCjIFQvMC_c/V0U9qFiMa_I/AAAAAAAAAks/uKDZ7AQ3Eqw11UY6L6E7pVO62JouTJCfgCLcB/s1600/musket_bess+2.jpg

you ripped the pre-made paper cartridge in your teeth, poured the powder and shot into the gun, cocked the gun, and pulled the trigger.

one advantage the musket had, is that with the bayonet it is a 6 foot long spear

the range of a musket was very long, much further than a longbow, but the effective range was probably less. a musket can shoot about 100 yards accurately, but the english used to practice longbows out to about 200 yards.

One thing that needs to be made clear in comparing bows and guns is the difference between knowledge and ability. Compare to a bow, a musket is more complicated to operate for the reasons already stated: the numerous steps involved in safely loading, shooting and reloading a musket are a whole new level of complexity compared to nocking and loosing an arrow.

However, these skills can be taught and learned within months, requiring no prior knowledge, skill set or conditioning. The operation of the musket was performed to a set pattern and trained as such, either by verbal commands, instruments or "by the numbers", and the very purpose of "drill" was to hone these steps into muscle memory and discipline.

Shooting a bow, on the other hand, required years of physical conditioning and practice to be proficient. To shoot the bows used in war, typified by very high draw weights (100lbs and higher), the archer needed to reach and maintain and peak level of physical conditioning and be capable of shooting accurately. Armies that had strong archery traditions either required them as part of their daily life (e.g. Mongols) or mandated them through training the entire civilian population (e.g. England).

Donald Featherstone (The Bowmen of England) writes:

...the crucial factor was that the archer had to be an athlete in the best physical condition, whereas the man behind the gun could function even in the state of weary debility produced by the cold, wet and hunger of extended active service...
Perhaps regrettably, today it is only the incurable romanticist who will claim special virtues for the longbow as against firearms. But, in the end, he will grudgingly have to admit that the firearm has proved to be what the bow could not become - a perfectible weapon.

That ultimately highlights the weakness of the bow as a weapon - archers were a very specialised force that was not sustainable and, when numbers were depleted, took a long time to replace. When presented with a weapon that anyone could be trained to use in a fraction of the time, and with much more effectiveness, it was easy to see why guns were adopted over traditional weapons across the world.

Basically, the "strain" of using a musket is alleviated through training. The strain of using a bow can only be overcome through years of training and experience, and with little benefit over the firearm.

There are other reasons why firearms were quickly adopted and preferred over bows. Briefly:

• Effectiveness against armour: bows struggled against the development of plate armour, firearms led to their decline
• Economy and ease of manufacture: firearms and ammunition was easier to manufacture in large amounts, whereas bows and arrows were always an artisanal industry.
• More available manpower: as anyone could be trained to use a firearm, it was possible to train and mobilise more troops.
• More tactical versatility: wet weather might impede the use of firearms, but they will still function, while bows and arrows are severely affected by bad weather. Furthermore, a musketeer will load and fire their weapon with the same effect regardless of their physical condition. An archer will be unable to shoot effectively if they are hindered by injury or poor health, and fatigue will slacken their ability to shoot accurately and with the same power as the battle goes on.
• Greater range: while military doctrine kept muskets in relatively close distances, one must also remember the role of artillery. With artillery with greater range opening the battle, and the musket for closer distances, there was no room on the battlefield for archers.

While you're waiting on an answer, this post from /u/hborrgg should shine some light on the English perspective on bows and firearms.