UPSC can be pursued by any graduate from any field. There is not much of an added advantage if you do bsc Or ba Or engineering. Since there is a subject of your choice,  every graduate can choose there graduation subject. So if you are well versed with your subject, it is the same for all streams. Difference comes when you are studying during college,  as engineering,  medical fields demand a lot of time,  you will not be able to devote a lot of time towards studies and this may cause a disadvantage. Hope this helps