"hard to merge" is correct and usable in written English.
You can use it to describe something that is difficult or challenging to combine, such as merging two companies or two incompatible ideas. For example, "The two companies had a very different culture and structure, making it hard to merge them."
Information technology services and software companies are typically hard to merge because of either people or technology integration issues.
Consumers see it in things like Apple's iTunes, which is hard to merge with other online music systems while retaining the same quality of service.
In reality, people are spread across a number of locations, databases can be hard to merge and most of the tools that companies use require significant training.
But the limited financial support, as well as technical problems that make it hard to merge registry information with Medicare records, have undermined the registry's utility.
Different data formats can make it hard to merge data.
Nokia is working hard to merge mobile phones with mobile computing, making it easier to access and use of the full suite of Internet resources from a variety of devices.
The Sprint and Nextel networks operated on different wireless technologies, which made it harder to merge operations.
Oh and this time, Warner would find it harder to merge with EMI publishing – because it would create a company with 34% share.
Were they to merge, it's not just that the corporate cultures would be hard to blend.
The prospect of its elimination aroused fierce opposition from many businesses, which argued that changing the merger rules would make it harder for companies to merge, and legislation was introduced in Congress to stop the board from acting.
"We are going to fight to allow regulators to break up big companies if they're hurting consumers and to make it harder for companies to merge if it reduces competition".
