If you ensure that the seller is the river itself rather than a third party then I think you can regard it as genuine. I can’t see them selling counterfeit items themselves.
However I bought a Kingston card from them in October last year and it failed in June this year. This is what they said when I followed up with them.
“As you’ve had satisfactory use of this product for a period of time exceeding both the manufacturer’s warranty period and the typical average life of this product, and since you purchased your product approximately 8 months ago and have used it successfully, please understand that we’re unable to offer you a replacement or refund of the replacement value”
They wouldn’t budge until I escalated, even when I sent in the online chat the link to Kingstons warranty page stating a 5 year warranty and it had to be progressed through the retailer. This has made me wary buying from this retailer now, it’s the first issue I have had but it required me to become very robust on a follow up to affect a refund.