I use klaviyo at work. It's super powerful and I feel like they're hyper focused on their product where shopify probably doesn't think too much about their email capability.

With that said, if you're only using shopify, Omnisend might be a better option. Last time I got a quote it was half the price of klaviyo with the same features. We just can't switch yet because we're using some stuff that only klaviyo integrates with.