Here are some pros to using this type of army list. First, the enemy is highly unlikely to have sufficient tools to counter your units. When this occurs, you can piece-meal your opponent's army and avoid those that can hurt you. Alternatively, you can full out crush the elements in the opposition's list that will cause you headaches, then allowing you free reign the rest of the battle. Of course, the strategy you choose will ultimately depend on how mobile your force is. The next pro ties into the first and it is toughness. In a rock army, toughness is what you can count on to get you to the end of the battle. This might be in the form of high armor values, a good armor save, or even a high actual toughness stat. With any of these items, you can effectively camp an objective with little to fear, assault vulnerable units, or eliminate like-wise tough units. Using your toughness, you can reliably deny your opponent kill points, while presented limited kill points for them to kill in the first place. Lastly, rock armies are extremely simple to play. They are often just point and shoot.
Now to the cons. Unfortunately, these armies are often a one trick pony. Because of their limited numbers, these armies will face a significant challege to overcome armies that are naturally good at killing that type of army (i.e. Dark Eldar disintegrators against terminators). If you are facing your natural predator army, you will have a difficult time beating them with your limited resources where your toughness does nothing to protect you. Another downfall in regards to limited numbers is the fact that those armies are also limited in the enemy units they can engage and fire at. This might mean you do not have enough firepower to eliminate enough of the opponent's force or control enough objectives to win the game. There is also a big drawback of being effective against your current opponent. For example, a Land Raider list would be extremely ineffective against a mechanized Imperial Guard list.
Now that you have seen some of the advantages and disadvantages of using these rock armies, you can make up your own mind regarding what is an appropriate army for you to use. See you on the battlefield!