Attempts to merge two major national unions can be extraordinarily difficult, and the current battles between the Screen Actors Guild (SAG) and the American Federation of Television and Radio Artists (AFTRA) illustrate why NEA and AFT have not revisited their failed 1998 merger attempt.
The boards of directors of SAG and AFTRA approved a merger plan, received the endorsement of big-name Hollywood stars, and is placing the unification before the rank-and-file for a vote (something NEA and AFT did not consider).
Ultimately these types of issues have less to do with the relationship between the two involved unions, and more to do with the relationship between the union leadership and their respective rank-and-file members. If the members trust their leaders, they will approve the deal. If not, they will vote it down and probably try to parlay that vote into an opposition slate for union office. It will also greatly reduce the merger option as a viable alternative for other unions.