1. You have raised a valid concern. Since the drain pipes of neighbour runs through your land, then in the even of any plumbing issues, your land will be affected.
2. This also implies that you are giving easementary rights to your neighbour and similarly you have an easementary right over your neigbour's land for purpose of doing any plumbing repairs.
3. This also means that you will be at the mercy of your neighbour should the gate be locked or your neighbour refusing to give you access to his land.
4. This will rather be problematic in the long run. Though on the flipside it cant be taken that there will be any imminent or immediate plumbing issues but the possibility cannot be ruled out.
5. So you can either request your builder to change the layout for laying the drain pipes or let it remain as it is and demand a reduction in price or simply exit from the project and take refund of your money or request the builder to allot you a land in the project layout which does not have the present issue.
6. Its completely your call.