If you look at CNBC video on the deal (https://www.cnbc.com/2018/03/12/us-tells-broadcom-it-confirm...) which talks about the Government concerns there are two parts:

1) Its a foreign company takeover. This will not hold as Broadcom is domiciling in the US soon.

2) Broadcom has publicly stated that it intends to do private equity style management. They are going to add $100B in debt - which might make them cut R&D. For its size, Qualcomm spends significantly more on R&D and has been a pioneer of many technologies. CFIUS contends this is a natural security concern. Any reduction in US R&D spend on key technologies like 5G will lead to China filling the void.