This review provides the current body of evidence on the X which will assist researchers in designing future studies.
2- However, the variability in indicators utilised in assessing the X across studies made collation of study results difficult.
A 'review' is a critical appraisal of a research article, or subject area. You are promising to 'provide the current body of evidence' on X, presumably a topic in a subject area (- at least, not a single article.). How? This is either some vast undertaking of collating all the published studies into one volume, or at least an exhaustive Reference of the published studies - and that's not your intended meaning in the sentence. That part needs rephrasing; or you need to change the word 'review'.
However, the variability in indicators utilised in assessing the X across studies made collation of study results difficult.
Not knowing what X is, means I am unable to comment on whether a 'the' is needed throughout.
It sounds like you a doing some kind of meta-analysis. When you write:
"made collation of study results difficult"
...I think you mean, "made (direct) comparison of research findings difficult" so that a meta-analysis could be performed.