Yes, that's true. As for whether it is serious or not, it depends on the degree of bad feng shui in the grave. If it is serious, future generations will be plagued by disasters, and even the family will be ruined and there will be no fragrance to continue.
If you want to know more, you can go to the professional Feng Shui website-Hubei Institute of Yin and Yang Geography, which has an explanation about Feng Shui. I believe you will benefit a lot.