A: The only time you hear about acquisition integration problems is when a company misses Wall St. expectations (see ""). To some degree, it can be a catch all as far as excuses go, but I think that most of the time the issues are much worse and much more ubiquitous than we know.
First, let's look at the basics. One company acquires another company for only a few reasons: they need a product or technology to complete or enhance their offerings into their existing customer base; they buy a company that extends the customer base; or they buy a company as a financial investment designed to diversify their overall portfolio. There are always other motivations (i.e., greed, stupidity, arrogance, nepotism, etc.), but these cover the bulk of the legitimate ones.
The second consideration is whether the acquirer is smaller than the buyer (typically) or about the same size. Sometimes it even happens when the one bought is bigger, but that only happens when something is very, very wrong.
The third consideration is whether to integrate the acquisition, or to leave it alone. You integrate the company you buys when they either have some products or technology you want. Now that you own the technology, you probably want the engineers who create and support it, but you can probably get rid of most of the other folks as they are redundant, thus lowering the real costs of the acquisition. You might pick up some talent at various levels along the way, but to integrate them means you are looking to get rid of redundancies and create economies of scale for the business you now own. If business A buys business B because business B has a great product and $50 million in sales, business A, which has $500 million in sales, can probably buy things cheaper, make them cheaper, distribute them cheaper, and so on. This makes business B more profitable and then business A fires the overlapped folks from the overlapped departments to make it even more so.
If you elect to leave it alone, you try to pass on as much of the savings you can letting business B leverage your size with finance, suppliers and distribution, but you tend to keep most of the folks and let the company run as a semi-independent. You get less economies of scale, but you tend to screw it up less if it was ok to begin with.