Dogs as they are implemented right now are unfit for gameplay. Which is why we're doing our best to develop new versions of the dog.
It is true, as Verti and Coolrock put it, that the dog is extremely fun. At least for the first few games. You can run around, woof, and bark at enemies and kill them. But after a while, you discover that they are not as fun as one might first believe. This engine has real trouble when it comes to melee weapons. The dog has had a few internal balance changes already, but one of the changes fixed the issue with the unit.
Currently, a dog either dies (unlikely to happen due to its deserved speed and its size) before it can get to you, or a dog closes in (which is usually the case) and initiates phase two. Phase two of dog gameplay is to bite the enemy infantry to death. However, due to the way infantry units and melee weapons are handled, both players will start jumping around to evade either the bite or the bullets. This results in a knife fight where players jump around while the dog spam clicks the attack button while the infantry unit sprays and prays. This is, after a few times, generally considered to be unfun and most feedback supports this. Even then, most times, the dog outright wins (which should be ok in many situations).
However, that is not all. The initial version of the dog didn't have an AOW attack. Rather, it had a specific point where the bite would land. This caused issues with units such as the GI and Guardian GI, which can deploy and, when possitioned correctly, be became invulnerable to dogs, as dogs couldn't get close enough to bite. This was solved by the introduction of an AOW effect to the dog. This, in turn, meant that it was pretty much GG for any infantry the moment a dog got close, which it generally did. We lowered its damage output to see if it would balance the dog. Let's just say that it didn't. Infantry, even expensive dedicated anti-infantry ones continued to drop like flies after the dog-infantry knife fight whenever a dog was part of combat.
This, however, isn't the only issue with the dog. For example, the map RA2_DunePatrol (soon to receive a massive update, you heard it here first!) doesn't have any base defenses. There's always a player on Dune Patrol that immediately purchases a dog, which is then used to rush the enemy base with. Once there, it starts a rampage killing all infantry in the base, including ones that have just spawned. Due to its speed and attack power, the infantry generally don't stand a chance, which causes a ton of disruption and player deaths. It takes a good coordinated squad to take one down and even then dogs are capable of simply eating through all defenders inside the buildings. This is a ton of direct and indirect damage for a 200 credit unit and even low-skilled players are capable of pulling it off repeatedly. This especially has caused a lot of frustration on those that are on the receiving end. It has also become the go-to unit for clearing your base from infantry. A few GIs or a Tanya that have managed to get to the weakpoint of your warfactory? Why buy three desolators or a Boris to fight them when just one 200 credit dog can deal with the invasion without problems?
Most feedback from the people playing the dog acknowledge that it breaks the game in its current form, and that is despite it being useless against buildings and vehicles. We are working towards introducing multiple new dog concepts that will hopefully turn the dog into a balanced and fair addition to the game. We will test all of them but the current version will be shelved.