I feel like Im being trolled here, especially with the username Test12345 but whatever I'll answer it anyways.
Its not a downside issue, its a choosing the right lnb for the job issue.
Do you want Cband ? get a Cband lnb
Do you want Kuband ? get a Kuband lnb
Do you want C and Ku Band ? get a C and Ku band lnb
Now there is alot of reasons why, but without getting technical a combo C/Ku lnb will always perform slightly worse then a dedicate C band lnb on a C band dish, and a dedicated Ku band lnb on a Ku dish. But of course that takes more room to have two dish's vs one. So many of us, like me, have a combo lnb so we can only use one dish vs two.
UDL