The phrase "mostly effective" is correct and can be used in written English.
It is typically used to describe something that is effective to a large extent, but may have some flaws or limitations. Example: The new marketing strategy was mostly effective in increasing sales, but there were some areas that still needed improvement.
To determine toolkit effectiveness, Lugtenberg et al's 20 method was adopted to assign outcomes from each toolkit to one of three categories: (1) not effective (if no significant effects were demonstrated); (2) partially effective (if half or less of the outcome measures showed significant effects) or (3) mostly effective (if more than half the outcome measures showed significant effects).
This template ranks dissemination activities or tools from mostly effective (for example, educational outreach, combined interventions) to unknown effects (for example, arts-based KT, social media).
Still, the performances are mostly effective.
I found his work fresh, provocative and mostly effective.
The Mets' bullpen had a mostly effective game.
Here, John De Lancie delivered the minimally smart-alecky and mostly effective spoken text.
In particular, the evidence to date suggests that training and private sector employment programmes have been generally effective, but mostly in the medium to long-term, while in the short-term effects are negligible.
IBD testing concerns mostly the effective population size in the past G generations, where G = 25, for example (the recent effective population size), whereas SNP association testing is concerned with much larger timescales (the long-term effective population size).
The Giants' defense had mostly been effective in keeping Garcia in the pocket in the first half.
Roger Clemens, Pettitte and Mike Mussina have mostly been effective in their starts, and Orlando Hernández allowed one run in eight innings against the Seattle Mariners on Saturday, in what was his best start of the year, by far.
In contrast, considering classification tasks with similar degradations in the training and the evaluation set, non-invariant image descriptors have proven to be mostly quite effective [21].
