Its less good for setting up when you cant immediately capitalize on whats in the yard
Welcome Boosters are free at stores, have identical contents, and contain legendaries, planeswalkers, and Showcase Cards
Also, there are situations where you dont want to trigger Prized Amalgam, for instance, you play around Ugin minus ability or vs Anger of the Gods and you have Silversmote Ghoul/another Amalgam triggered to return at your endstep, or fetchland + Bloodghast ready to return your 3/3s on the opponents endstep
That's a lot of money, especially when considering that a booster pack of Magic cards sells for between $4 and $10
Unlike the cheaper tribes, which need plenty of cards of that type to function, simply casting Silumgar into Atarka will tend to put the game right into your grip