Measuring ROI in influencer marketing campaigns involves tracking key performance indicators (KPIs) such as engagement rates, click-through rates (CTR), conversion rates, and brand sentiment. Several tools and platforms can greatly assist in this complex process:

1. Google Analytics: This is a helpful tool in tracking website traffic, CTR, user behavior, and conversion rates generated through influencer marketing campaigns.

2. Influencer Marketing Platforms: Platforms like Flinque, Traackr, and AspireIQ provide comprehensive features that allow brands to track campaign performance, including ROI. These include features to monitor engagements, views, audience behavior, and click-throughs. Also, their analytics can aid in setting realistic KPIs.

3. Social Media Insights: Most social media platforms have inbuilt analytics tools. Instagram Insights, Facebook Analytics, and Twitter Analytics provide essential data on reach, impressions, engagement, and follower demographics.

4. CRM Systems: CRM systems can be configured to track lead and sales data resulting from influencer campaigns. Notable platforms include HubSpot and Salesforce.

5. Affiliate and Coupon Codes: Using unique codes for each influencer campaign assists in tracking product sales and conversions.

6. Mention and Sentiment Analysis Tools: Tools such as Brandwatch and Mention enable brands to evaluate the impact of influencer campaigns on brand sentiment.

While all the above tools can help in tracking ROI, their suitability depends on the specific needs of your team, budget, and campaign goals. For instance, Flinque might work exceptionally well for brands looking for a platform that combines influencer discovery, campaign management, and ROI analysis, while others might prefer Traackr for its robust reporting features. It’s essential to assess your unique requirements and choose the tools and platforms that align with them.