Now that New Leaf trading has slowed down a bit, the amiibos and the Welcome Amiibo card pack is worth it if you want to obtain items on your own without trading with others. The WA cards give you a whole bunch of new items that you wont find at the shops (you purchase from their RV in the campsite), plus you can invite those villagers to live in your town. There are also some special characters you can invite to New Leaf using the figurines, like the Legend of Zelda villagers and Splatoon villagers, but those aren't available to New Horizons.
Amiibo cards in NH are mainly for inviting villagers to live on your island or to design houses for them on Happy Home Paradise (if you are impatient about waiting for them to show up naturally). You can't order items from them like in New Leaf, instead, they will ask you to craft items for them after giving you a DIY card. This is useful in early game days to get extra DIY cards.