It’s really easy to spot effective vs. ineffective/“empty calorie” growth channels when you look at them through the lens of a single word - intent.

Ads appearing alongside search results are among the highest intent, which is why they perform well and are thus so expensive.

Referral programs and coupons are people looking for generic deals. You might form habits among a few of them, but the likelihood is low. Free trials are probably more dependent on the context.