The hardship standard for the I-601 and I-601A are identical. The important difference is that the I-601 applicant must apply from outside of the United States and must demonstrate extreme hardship to a US citizen or permanent resident spouse or parent and the I-601A applicant may apply from inside of the United States and must demonstrate extreme hardship to a US citizen spouse or parent. USCIS will not consider evidence of hardship to US citizen or permanent resident children unless it is explained in a way that demonstrates that it effectively a hardship to the qualifying relative.
There is no limit to the number of I-601 and I-601A applications a person can file. But waiver applications take time and money, so it's wise to carefully prepare the application.