For Space Marines and their various chapters this strategy works very well. Marines with combat tactics are even designed to take full advantage of it. Even so, there are reasons that many players have a hard time with it. Some of the disadvantages come with having smaller and more vulnerable units. Literally they vanish much faster off the table with less numbers. I keep seeing this over and over with people playing the new Dark Eldar codex. Small units of trueborn, wyches, etc getting hit by rapid fire bolters and getting slaughtered.
One of the primary reasons people are losing these small units is target saturation. There is the false belief that just because I throw a lot of small units out there that I will overwhelm the enemy and they wont be able to shoot at everything. While this works with newer players, an experienced player's units generally have a very distinct job on the battlefield.
An experienced player is often coming to the table with a force ready to take on infantry, Light armor, heavy armor etc. Each unit has a target type that they normally kill extremely well. For example, Chimeras with a Lascannon firing out of the hatch. The chimera is exceptional at killing light infantry, while the lascannon excels at bringing down vehicles. If you bring a mixed force coming at a list with many chimeras and lascannons, you can expect the chimeras to really mow down your troops and the lascannons to bring down your vehicles. Dark Eldar with very few raiders, and small infantry units in the void of a game table, get mowed down pretty easily. I've seen this a lot.
Target saturation is not just simply having a lot of units on the board. It's about overloading the number of targets that fit into a category so that there are simply too many targets for your enemy to take out. Example, Blood Angels overloading their list with heavy armour. If there are more targets than your opponents lascannons can deal with, you will overwhelm him. It is one of the reasons chimera chassis armies do well, they overload the opponent with medium armor.
Ranges can do this type of target saturation as well. If your opponent only has a couple weapons that can deal with you at range and you out range him with most of your army, it will be a slow slaughter.