It is an open problem in the literature to derive optimum tests for the equality of treatment effects in an unbalanced two-way classification model. For such models without interaction, optimum tests are derived in the following cases: (i) the locally best invariant unbiased test for the random effects model corresponding to an equiblock and equireplicate design, (ii) the locally best invariant unbiased test for the mixed effects model with mixed treatment effects corresponding to a balanced incomplete block design and (iii) the uniformly most powerful invariant test or the locally best invariant test for the mixed effects model with random treatment effects. Robustness of the optimum invariant tests against suitable deviations from normality is also indicated.
Thomas Mathew. Bimal Kumar Sinha. "Optimum Tests in Unbalanced Two-Way Models without Interaction." Ann. Statist. 16 (4) 1727 - 1740, December, 1988. https://doi.org/10.1214/aos/1176351065