This may be a long one but bear with me:

One of the challenges with working with espresso starts with the equipment used, namely the grinder and espresso machine. Higher quality grinders provide consistency and homogenous grind particles which are necessary to deliver a consistent shot, this is far more critical in espresso making than it is in any other brewing method (though it is still important in all other brewing methods). Higher quality grinders mean more consistency and less effort in order to adequately dial in and pull a shot of espresso, whereas lower end grinders may have issues regarding build up, uneven coarseness, changes in grind/dose shot-to-shot and other issues which the operator must anticipate and react to. Having used a few different grinders at different cafes made me realize that the difference between a $1000 grinder vs a $3000 grinder is HUGE in this respect.

The espresso machine also plays a role in skill involved especially with a plethora of variations in equipment from manual levers to pressurized gaskets to "pressure profiling" (more like flow regulating) machines. Water temperature and pressure control will vary between machines and being able to understand these changes and how they will affect the shot requires some knowledge. There are also issues regarding consistency whether it's temp control or pressure control; though this is mostly dependent on the quality of the machine it still means the operator needs to know what is or may happen and how to adjust for it.

The coffee itself is a whole other beast. Age is an important factor in coffee but how the coffee ages will affect the results more in espresso than it would in other brew methods. Constant calibration and re-calibration of grind/dose/time levels for shots is necessary in order to produce a decent shot and the parameters produced on one day may not necessarily be repeatable another day due to age or moisture levels or how external variables such as humidity can affect the result. While all this is also factored in in other brew methods it is not as significant. At home I use an aeropress and Hario pour over and most of the time I do not need to sway away from my usual formulas to create a decent cup of coffee, with espresso on the other hand I may have to adjust my dosage or grind (or both) several times a day in order to achieve what I believe to be a decent shot. This is something most home users don't want to do because in a cafe setting dialing in and recalibrating requires a lot of wastage.

All of the variables in extraction for pour over (dose, grind, distribution, water volume, water temp, flow rate, total brew time, total extraction) are also part of preparing espresso. Unfortunately most of these factors can only be changed before or after a shot has been prepared which makes this a very tedious trial and error process, whereas with pour over things such as flow rate or brew time have (to some degree) a level of flexibility dependent on the user. The other issue is that since espresso is a tiny, volatile, emulsified drink the fluctuations in variables are exasperated greatly; many of my customers have very discerning tastebuds with regards to espresso and have returned drinks simply because of very very minor variations (eg. pulling a shot .5s longer than I should have or losing some of my dose during distribution). Other brewing methods have a bit more forgiveness in that regard.