There are a lot of people that live by the MSU strategy. MSU stands for Multiple Small Units. It is an excellent overall strategy as it gives you lots of units which tends to bring in more firepower, more flexibility on the tabletop, table coverage, and more specialty units. Even in Kill Point games a skilled player is rarely hampered by having so many units on the board.
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.
MSU (multiple small units) are not king. Instead it's the understanding of how to build a MSU list to take advantage of your larger numbers of small units. The goal is not to make your opponent's more efficient at killing you. I have had people come at me this way, where every unit I had was able to devastate and rip apart my opponent. Where I had more than enough units to handle him piece by piece. My heavy weapons blew his tanks, my flamers killed his scouts, and my plasmas were decimating his terminators. My opponent list and play simply made my army extremely efficient at killing him.