EF Core: Union vs LinqKit for dynamic linq to SQL

Question

I was always using the LinqKit to implement dynamic and cooperative predicates. However, the last time I saw a colleague use the Union to combine dynamic queries. I have found it too strange and tried to compare it with the linqKit in terms of performance, I have found it also similar or sometimes faster than the LinqKit method (multiple executions are used). However, I am not sure about the efficiency of my method. Could "union" be considered a good method for dynamic query generation? I know LinqKit has a cleaner code writing methodology. I want to know what could also outperform the use of "Union"

UPDATE: Added Code Sample:

 var query = db.Cases.AsQueryable();

        IQueryable<Case> finalQuery = null;

        //open request for super admin

        //return cases list based on role

        if (true /*something*/)
        {
            IQueryable<Case> newQuery = null;

            if (queryObject.IsForChooseRelatedCase == true)
                newQuery = query.Where(c => true /*put criteria*/);
            else
                newQuery = query.Where(c => true /*put other criteria*/);
            if (finalQuery == null) finalQuery = newQuery; else finalQuery = finalQuery.Union(newQuery);
        }

        if (true /*something*/)
        {
            IQueryable<Case> newQuery = null;

            if (queryObject.IsForChooseRelatedCase == true)
                newQuery = query.Where(c => c.BranchId == CurrentUser.BranchId && (c.Status == CaseStatuses.DoneJudgment || c.Status == CaseStatuses.ClosedCase || c.Status == CaseStatuses.ObjectionRecorded));
            else
                newQuery = query.Where(c => c.Researchers.Any(r => r.ResearcherId == CurrentUser.UserId && !r.IsDeleted) && c.Status != CaseStatuses.Draft);
            if (finalQuery == null) finalQuery = newQuery; else finalQuery = finalQuery.Union(newQuery);
        }

        var data = await finalQuery.ToListAsync();