I personally don't think that using dumbells instead of barbells for any of Darrin's routines would dramatically impede one's potential for progressing, unless the main goal is to get much, much stronger in particular lifts such as bench press. I prefer dumbells myself, especially for pressing movements. But if you begin lifting more consistently and still don't see results, make sure you are following the principles lined out in his ebook. Such as: Are you training to failure in the right rep range on each set? Are you using the correct time regimen between sets and between workouts? Are you eating the right amount of calories and getting enough fruits and vegies? Are you remembering to get a good dose of carb/protein portion before and after workouts with very little fats during that time period? Are you getting enough sleep and rest? Are you keeping a log to accurately track you progress? Are you increasing poundages in small increments when you reach the rep goals lined out in the book? Are you staying mentally focused during workouts and exhibiting a positive outlook regarding your fitness goals?
If you are doing all of this and still not progressing, then dumbells may be slightly holding you back.
The only drawback I can think of is that, in some of the movements, e.g. incline chest presses, hang cleans, and OHPs, the amount of weight one can handle with good form is limited, because one cannot move as much weight to the starting position without compromising good form or one may be expending too much energy getting to that starting position, so that maximum force is not achieved during the actual movement.
For instance, if one is training with dumbells by his or herself in the flat bench press or incline press, he or she may not be able to get as much weight to the starting position as he or she will be able to press for the desired repetition range. For me, this is more pronounced for the incline press movement than the flat bench, because I'm fighting gravity more going from my hips to the starting position on the incline bench. All this means is that most lifters won't be able to use a weight he or she can press only 3-8 times if one is trying to increase strength. In my case, I can move, without risk of shoulder injury, enough weight for inclines that I can pump out about ten times and about seven reps for the flat bench press. If I want to use more weight, I must use a barbell.
This drawback can be overcome, however, in two ways. Number one: use an informed, trusted spotter with a plan on how to hand off the dumbells. If either the lifter or spotter, however, do not know how to carefully hand off the dumbells to one another , the potential for injury is greater than going it alone. Remember, with heavy dumbells, the spotter cannot grab both dumbells at once and safely hand them off to you because they would have to hold them in the same spot you would need to grab them, so it must be thought out how the lifter will balance one dumbell while waiting to receive the other. If the spotter is holding only one dumbell, he or she can grab it on the ends and carefully hand it off in the desired position.
The second way to overcome this drawback is to do your heavy lifts on movements that are not limited with dumbells and use less weight for lifts that are.
For instance, I can use a chair or flat bench below a dip apparatus; strap on a heavy dumbell with a good, leather weight belt while on my knees or on one knee. I then stand up and put one foot in the now-empty chair. I then push up with both hands on the dip bars while simultaneously letting the lower foot go airborne. I then lift up the foot that was in the chair and bend at the knees so as not to hit the chair during the eccentric part of the movement. I'm now in the starting position. After I pump out that last rep, I simply bend at the elbows; straighten my legs; and reverse the order for my feet. If the dip bars are too high, you can simply drop down to the chair and then to the floor. Lastly, I bend at the knees and put the dumbell back on the chair; and then unstrap. Chinups and/or pull-ups can be accomplished using this same concept. As long as you do these steps methodically, use heavy, stable equipment, i.e. chair that won't tip forward, and make sure the dumbell is never placed where it can roll off and hit you or equipment, it's safe to do these weighted movements, even alone.
While working out at home, I learned how to do these weighted dips by myself safely. I thought of this after I got up to doing over twenty repetitions of dips and twelve reps of chins. Truthfully, I should have started doing these weighted much sooner. I have strapped on enough weight on both chins and dips where I could accomplish only 2-4 reps, so dumbells work for any rep range on some lifts.
The good thing about using dumbells, is that it challenges one's core and uses more overall muscle for some movements and therefore keeps one more balanced.