Leadbelcher units only have a range of 12 inches, not only that but will have to be stationary for a whole turn to reload before they can fire again.
So, what does this mean ? Well, for starters, a smart enemy will engage them with cheap skirmishers or fast cavalry to use up their stand and shoot charge reaction and then keep them engaged with other cavalry. This means that they will only get to shoot once, possibly for the whole game.
Also, there is a problem with shooting. On average they will fire 6 str 4 armour piercing shots (-2 armour save). That is, if a cannon doesn't misfire ! A misfire, causing D6 str 4 hits will on average kill an ogre and maybe wound another. There is a 1 in 6 chance of this happening for each cannon so if you have 3 ogres you have a 50% of losing one of them when you fire, this will seriously impact their combat effectiveness because they will be much more likely to be outnumbered in unit strength.
Also, because the range is so low and they only have light armour you will need to shield the unit until it is within range (this alone is quite difficult to do) and then maneuver them into position (even more difficult when the enemy has his own plans).
In summary, you are spending about an extra 20 points for the cannons. You need these Ogres to fight aswell because the cannons alone will not do sufficient damage to large rank and file units. To make sure they will be a viable fighting unit you would need to take 4, for a whopping 220 points. This is simply too much, use the points to bolster other combat units instead or get another Gnoblar Scraplauncher.
Well, that's my opinion anyway. Other people probably include them in all their Ogre battles but do they win their battles ? ;-)
