Standard cost-effectiveness methods and critical appraisal toolkits may not be adequate for complex interventions. We systematically reviewed and quality assessed cost-effectiveness studies of a complex intervention, propose a series of new questions to inform their critical appraisal and discuss how future research should be targeted to improve the methods. Reablement was used as an example of a complex intervention. Reablement is a multidisciplinary and multifactorial intervention to support people to relearn activities of daily living.  The systematic review identified 12 cost-effectiveness studies on reablement, out of 3,311 unique records. The 12 included studies were data extracted and quality evaluated using a standard checklist. No study provided enough information to inform the decision on whether reablement is cost-effective and should be reimbursed by the payer. The issues included: (i) the use of a perspective not relevant for the decision-maker, (ii) lack of consideration for inter-sectoral effects, (iii) short time horizon, (iv) poor descriptive detail on the interventions, (v) limited comparators, (vi) poor quality evidence on effectiveness, (vii) limited evaluation of uncertainty and (viii) no consideration of the opportunity cost. These issues informed the development of a new checklist, which was subsequently applied. Critical appraisals of cost-effectiveness studies should consider the aforementioned issues to conclude on their quality and potential to inform decision-making. More research is needed how to quantify the opportunity costs of complex interventions, particularly when multiple sectors are affected.