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If you don't sight the traffic and manouevre horizontally based on the picture being given to you by your kit, don't be too surprised if you get a nasty surprise on occasion.
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and for every occasion that the manoeuvre takes me closer to the other aircraft, there are 99 occasions when it really does increase the separation.
They are in effect the same thing, i.e sometimes the information will not be accurate in the horizontal plane, although you give a numerical value (backed up by a report somewhere I guess).
ACAS is nothing like a controller since they are trying to provide different things. A controller is trying to meet separation standards, which are large. ACAS is monitoring other traffic with a view to preventing a collision only. It won't care what your separation is, just as long as you manouevre out of the impending collision. It will also wait until an aircraft is potentially going to be much closer than a controller (in perfect circusmtances) would before giving a warning.
It is the last safety net you have left when ground based systems and your own eyes haven't worked. But it's still only designed for vertical resolutions. Being aware of the limitations is an essential part of the 'training' before using such kit in anger. But then no one insists on training apart from the large aircraft operators.