I’ve experienced it in the Banking sector and there certainly was a huge culture clash between both organizations once the post-merger integration phase was kicking in. This was honestly more on an MD and Global Head level and less so on the Director level. The main issues were around aggressiveness of target setting for new to bank clients and revenue generation. In addition, other factors such as product teams (treasury, trade finance) collaborating and working together by sharing client insights as well as team dynamics (going out for lunch, sports events on an org. level).
I work in the medical device industry, so I think it is very close to the pharma industry. My company (a Japanese company) purchased 2 German based start-ups in the past, and it ended up with a failure if I need to say from what we have seen after the acquisition. One of the clear reasons why it did not work out as expected is, without any doubts, company culture ( associated with general cultural differences between the countries) was too different. In my company, the decision making process requires a lot of efforts because every subtle things should be considered before the decision is taken, and many internal politics are involved to get consensus among the top management. The people at the HQ tried to manage some decision in the same way in the 2 acquired companies from the day 1, and the people could not understand at all why it changed that way. As the result, many important who like flexible decision making process left the companies.