"Successful M&A activities occur mostly when the target is a lot smaller than the buyer. This means:
1. The buyer can flood the target with its own staff to impose its corporate culture;
2. The buyer can spend more money to cope with unforeseen issues; or
3. The buyer is only interested in a key technology, a key market, or a key customer, and is willing to write off the rest of the target's business.
Mergers of equals seldom work because of ego and wallet issues. The target's top managers are not willing to get demoted, the target's corporate culture doesn't fit well with the buyer, the buyer doesn't have enough spare money to fix the unforeseen issues, etc.
There are many companies that have pulled off successful M&A exercises. Any time you see an announcement that the company has purchased another company for $X, and the transaction does not require shareholder approval, that's one M&A that is likely to work out. Too many such cases to count, really. But if the target is big enough to require shareholders' approval, it's probably also too big to pull off."