I've run a few A/B tests on this exact question - Test A being a dedicated affiliate network (Impact + ShareASale) versus Test B, an influencer relationship platform (Grin + manual tracking). The heatmap data from those sessions shows there isn't a single densest network, it's all down to your funnel stage.
If you're testing creators who already know affiliate mechanics, Impact or ShareASale give you a much larger pre-vetted pool. For DTC/CPG I've seen faster ROI because discovery, contracts, and payouts live in one place. Mid-sized creators already have profiles there - less friction than starting from scratch.
For influencer-style UGC (TikTok, Instagram) I've found Grin and Aspire work better for relationship management, but you still need a separate tracking backend like Refersion or Shopify Collabs to attribute conversions properly. That's where most of my tests have fallen flat - people overlook the tracking layer.
In pure e‑comm, the standard stack keeps coming back to Shopify + Impact or Refersion. I've got a few screenshots from a recent test that showed a 15 % lift in attributed revenue when we ditched the spreadsheet approach. Happy to share if it's useful.