India is a culturally divided country, especially along religious lines. For things like property rights, legality of marriage, and other civil matters (as opposed to criminal law), citizens are subject to different sets of laws dependent upon their religious affiliation, or other community designation.
The UCC is a proposal to establish a single set of laws for all citizens and residents of India, regardless of religion or community; essentially bringing their civil code's jurisdiction in line with that of their criminal code.
The benefits of this are numerous but there's three main points:
- Simplicity - A single set of laws makes determining what laws apply to a given person much easier to determine, and everyone can go about with the same set of expectations.
- Sovereignty - As it stands now, civil law in India is dependent upon religious traditions, rather than the authority of the Indian government. Establishing a UCC reinforces that law is the domain of the government of India, instead of religious authorities which may or may not draw influences from foreign lands.
- Anti-discrimination/Anti-oppression - A big selling point for the UCC among its proponents is that this will help to outlaw and curtail oppression of vulnerable minorities who, because of their religious affiliation, etc., are currently not entitled to civil law protections, despite being full Indian citizens.
The downsides are largely dependent on who you are and how well the status quo is working for you, but a period of legal uncertainty and instability will result as would happen from any major change in the legal landscape. This new system will take some getting used to.
Most other democratic nations, and a whole host of other nations besides, have something that looks like the UCC (though they don't call it that, per se). In most nations you have a single set of laws that applies to all citizens/residents, with occasional carve-outs for certain positions or conditions. India's civil law lassez faire is one of the relatively unique things about India.
As for why it is difficult, any major overhaul of an existing status quo is, politically speaking, a tall order. The fact that one of the earliest proponents for a uniform civil law was the British Crown likely doesn't help the optics here, either, however.