You get:
* integration technical problems
* power struggles
* dominance by the bigger group, and a lack of understanding what made the smaller company work so well
* some of your best people leave. If you mishandle it, lots of your best people leave.
* a major slowdown in any growth whilst largely valueless politics and questionable business practices distract the original company and they take their eye off the ball
Honestly, precisely how many tech takeovers really work? The CEO and board gets this great idea that they can buy a hot company to turn around the fortunes of their existing company. Then they take all their inefficiencies, bad ideas, politics, incompetence and total lack on insight and corporate memory and force that upon the company they bought. They normally chuck out the original management (not always, but frequently) who made it all work in the first place, then they start imposing changes on customers, leading to pissed off customers. A good example is that the taken-over company has support processes that work quite well for them, but they force customers to go through their existing support systems, which are often slow, inefficient, off-shore, and have a first point of call who probably doesn't know much about the service they are meant to support.
Often the parent company don't know about back channels used by the original company to gather feedback to iterate effectively. Those aren't normally documented and are part of institutional memory and culture, gained over years of hard work.
Goodwill in the company then dives, and after 2-3 years (sometimes earlier) the original management team have a shell of the original company. Then they realise that their profit forecasts they originally promised to justify the takeover were hugely optimistic, and even the realistic forecast that in all probability would have been met had they not taken over the company won't be met due to the bloody awful decisions they have made.
At this point, they either: roll back their pointless or bad changes, bring in a new management team, or shutter the acquired company after strip mining the assets of their acquisition.
Rolling back the changes often leads to an even worse situation because now the original company has lost their momentum, lots of key people have left, they haven't been able to adapt as the market changes, and they have lost a huge amount of existing customer goodwill. Now the company must a. Fix the problems caused by the people who imposed it on them, b. Still play political games as people who were responsible for the awful decisions cover their arse and still have the ear of those in the parent company (which makes it harder to justify going back to the way things were), c. Explain to customers who have been put through turmoil (and who might be getting used to the albeit detrimental changes they have experienced) why they must now go through yet more changes as the company tries to put things back to the way they were, and d. They now have to justify their very existence, even though their momentum, talent, and direction were severely and adversely disrupted. Point d. is particularly damaging because they are still expected to meet the unrealistic earnings targets predicted by the original CEO, but often it's an impossible task.
Bringing in a new management team rarely works. Unless they are extremely good, then all the disruption of rolling back changes tends to happen, but now you have a new set of people who don't have a clear grasp of the business, and this time they don't even have the advantages of the original team (advise, goodwill, talent, etc) that the original management team had.
Eventually, if rolling back changes or bringing in a new management team don't work, then the original company gets shuttered anyway. If the acquiring company is lucky, the profits generated by the acquired company largely cover the costs of acquisition. But they lose enormous amounts of goodwill, both from those who are retrenched (ultimately for no good reason) and who might be potential customers or even competitors in the future, and from customers who were treated shoddily and who quite possibly see your company as a risk to deal with as they were left in the lurch and now need to spend time and resources finding an alternative product or service, with all its corresponding risks and implementation issues.
I can't see larger companies ceasing or even improving their acquisition processes any time soon. All I can say is that if you are a customer who uses a product or service from a company who is taken over or is part of a merger, start immediately planning on the best way to migrate from them should you need to so you mitigate your risk. And if you are an employee, by all means hold on for the ride, but be aware that the things you love about working for your company are quite likely to change or be entirely lost - and ultimately you may find you won't have a job. Dust off your resume, do not fight the system (that way madness lies), accept it's out of your control and take steps to handle disruption in your working life.