Is product reviews safe as a link building tactic? Not if the links are followed.
If you offer influencers products for review in exchange for links, the links must be nofollow. Additionally, the influencer has to disclose the paid relationship as well (an FTC requirement as well).
According to Google,
“Links that pass PageRank in exchange for goods or services are against Google guidelines on link schemes.” – source
Giveaways are also a pretty popular tactic with online retailers, but the links obtained directly from offering giveaways have to be marked with the nofollow attribute.