Why CIWS Won't Work On Ground Vehicles
While CIWS systems are effective on naval vessels, there is a reason; they usually have a clear firing background and detect the missile from several thousand yards away. Even then, they still fail to intercept incoming missiles a fair amount of the time (the Harpoon game gives them a 80% success rate, which may be a little low).
Now take the CIWS on a tank. ATGMs are fired from much shorter ranges (500-3000 yards). Unless they are air-launched, your firing background will have trees, rocks, buidlings, other vehicles, etc. Your success rate will (or should) drop drastically. I once fell in love with CIWS mounts on tanks, but I've since realized the truth: they won't work.
A. Active Sensors: Forget it. You've just spent millions to make your tank harder to see (emission cloaking, stealth, chameleon, etc.) and you're going to put a beacon on top of it saying "slap another HARM missile into me please." Bad idea. If you make your active sensor very short ranged (say 0.1-0.2 miles) to make it harder for enemy forces to detect, the CIWS will *not* have time to react to an incoming missile. Active sensors can also be jammed.
B. Passive Sensors: Passive radar is basically enhanced thermograph, not a motion detector. It gives the CIWS computer a pretty picture and a specialized computer program (probably Complexity 3 or 4) must determine if any of the things it sees are are incoming missiles. Given the amount of time it will have before the missile impacts vs the rotation speed of the passive radar (once per second? twice? ten times? -- the program will likely need more than one pass to make a positive identifaction), the computer probably won't be able to pick the missile out as a missile in time unless it's slow and/or coming from a long ways off. Also, in the heat of battle, lots of things are going to be moving around the battlefield, possibly confusing the CIWS (this isn't a problem at sea). For example, a friendly SAM team fires at a helicopter on the other side of your tank -- only to have the CIWS think the missile is coming at the tank and shoot it down.
II. Attack Sources
A. Aircraft: If air-launched weapons are enough a threat to your armored units to justify a CIWS mount, then either you've lost air superiority or your ground-based anti-aircraft weapons are insufficient. In either case, you tanks will die quickly; putting a puny CIWS turret on your tank will not save you. You should spend the money instead on ways to knock out the enemy's aircraft -- cure the disease, not the symptom.
B. Infantry: Okay, the infantry are dug in, some 500-1000 yards away firing missiles that can cover that range in 2-3 seconds (or much less, for hyperkenetic missiles); are you going to tell me that that's enough time for the CIWS to spot the missile, recognize that it's a missile, rotate to face it and fire with any degree of accuracy? I don't think so. Plus, the missile is going to be coming in at 3'-4', so the CIWS will have to look down, adding ground clutter (and infantry) to it's detection problems.
A. Decieving the Sensors: Some armies may use remote-controlled rocket launchers, firing cheap, obsolete rockets (say, M72 LAWs or AT-4s) at CIWS-equipped tanks. This will bleed away the CIWS' limited (usually 20-40 bursts) ammo supply quickly, leaving them vulnerable later to the primary ATGMs. To keep you honest, mixed in with those sacrificial lambs may be a top-line missile, so if you've turned of the CIWS to save ammo -- whap.
B. Other Weapons: If ground-based CIWS are really effective, your enemy will start using other weapons that are much harder to intercept. One-shot railguns are one choice; autoloading recoiless rifles (in underwing aircraft pods) are another (think a 75mm's 5dx15(10) round won't go though your top armor? The range is awful but they're very cheap). And, of course, large autocannons can chew through your top armor as well. Robotic mini-planes (basically small RPVs), with a fair amount of sensor masking, may replace missiles in some situations.
C. Multiple Attacks: A robot-controlled system can easily fire missiles from two (or more) completely seperate locations simultaneously; your CIWS can only stop one. Aircraft (and some ground vehicles) can launch multiple missiles as well, overwhelming the capabilities of the CIWS (think of seven 70mm guided missiles at once). Cluster bombs and dispensers will overwhelm a CIWS.
D. Anti-tank CIWS: If CIWS on ground vehicles would work, then you could use robot sentries designed as static (and hidden) anti-tank weapons, armed with one-shot railguns, using the same technology (as soon as the robot picks detects a tank within a certain range, it makes an IFF query and immediately fires if the tank's IFF fails to respond correctly).
IV. Other Considerations
A. Hull Down: With a tall dome on the top of your turret, you can no longer assume an effective hull-down position (the CIWS mount is there for everyone to see). Being able to hide behind something while still being able to shoot over such cover is vital for tanks (or a soldier, for that matter); the less of you that is seen the better, which it pretty hard to do with a CIWS mount.
B. Friendly Fire: Dismounted mechanized infantry will severely dislike CIWS weapons firing over or through their formations, even if their armor DR can take it (if nothing else, it's a potentially fatal distraction to be hit by CIWS fire from behind). Normal infantry are likely to have lighter armor and so may be vulnerable to heavier CIWS guns. Side note: Combat Infantry Dress (the standard TL8 soldier's armor, according to Ultra tech) has DR 12 on arms and legs, which requires a weapon as small as 3mm to pentrate it half the time; not fatal, but not good.