The nexttransactionlinelink table is currently defined as non-incremental sync. However, it does have a last_modified_date column and key columns defined.
Queries against the table have shown that previousdoc/previousline/nextdoc/nextline when taken as a whole are unique throughout the table, and the NetSuite schema does identify the four as keys.
In the source NetSuite2 table, these four columns (as one surrogate key) are absolutely unique. In the Fivetran imported table, they are also unique, except for the last_modified_date.
It seems that if the four columns noted are treated as a surrogate key, this table would be a candidate to be converted to incremental refresh.
I am happy to jump on a call to discuss. I can also provide the queries I used to research the table.