I want to know if bringing the image of a DSO in eyepiece generally represents a problem for beginners in astronomy.
Some can, some can't. Viewing really is determined by ability, experience and technique when you leave viewing conditions aside.
A lot of DSOs have a lower surface brightness, one thing I always notice is when an observer continuously increases their magnification to see the object larger. Surface brightness is proportional to magnification, the higher the magnification the less bright an object appears, to see an object as bright as possible you need to use the lowest possible magnification. Using a smaller diameter eyepiece (larger magnification), you will need to develop techniques such as averted vision to truly see the beauty of some objects that otherwise appear dull.
Finding them can be exceptionally hard if your not familiar with the viewing region, easily overlooked or thought to be a tuft of cloud. Eye adaption is key when viewing so you need to spend significant time on an object before the details appear.
The Messier objects contain some well known DSOs, and can show some amazing details with patience. Understanding that the Messier objects were cataloged with poor optics and small apertures makes them prime targets for beginners.
It's a pretty broad topic. It depends a bit on the type of DSO. Speaking in general, most DSOs lack brightness, and so to observe them you need two things:
Observing from the city, DSOs are challenging. The further away you are from city lights, the better your views. Deserts, national parks, sparsely populated areas are all good. Driving at least 1 hour away from the city usually provides very noticeable improvements. Light pollution maps help in finding places with a dark sky.
The bigger the aperture, the better you can see DSOs. If you're a DSO hunter, you must be focused on increasing aperture before you do anything else. What helps here is having an instrument with a good aperture / cost ratio, such as a dobsonian reflector.
There's essentially no limit here - more is always better.
It should be noted that a larger aperture will always perform better than a smaller one, even when light pollution is very heavy - but of course it's best if both aperture and dark sky cooperate.
Other factors that may help:
These are the most over-utilized and over-hyped accessories, especially for beginners (most people are too much focused on filters and not enough focused on things that really matter). That being said, some nebulae do look better in some filters.
Dave Knisely is one of the foremost experts in the amateur astronomy community in terms of DSOs and filters. Read anything he writes on the subject, e.g. this long article:
In a nutshell: If you can only get one DSO filter, get a UHC. If you can get two, also get an OIII filter. Of course, dark sky and large aperture should always come first. Don't bother with filters if you're observing from the city using a small aperture.
Filters are useful for nebulae. They don't help with galaxies, star clusters, individual stars, etc.
Find a book called Turn Left At Orion. It's a fantastic introduction for beginners to DSOs. It will help you find the easier targets, and essentially open up the sky for you. Later you'll be able to figure things out on your own.
It should be noted that the entire Messier catalog can be observed in a relatively small telescope even in the city - and all Messier objects are DSOs. I've seen the Dumbbell Nebula (M27) and the Ring Nebula (M57) in 6" of aperture in a very light-polluted place (Silicon Valley). So, no, it's not that hard to observe DSOs.
Of course, if the sky is dark and/or the aperture large, all DSOs look better. This is especially true of globular clusters, such as M13. They already look pretty spectacular in any aperture, even in a small scope - but the view is mind boggling in a very large dob.