Overall, The Mechanist feels like an upgrade to Sai, Master Thopterist
They teamed up to fight the Eldrazi and the Consulate, got split up by Nicol Bolas during the events of Hour of Devastation , and came back together for War of the Spark
Combine them with special Land cards featuring artwork reminiscent of the original Middle-earth maps, 'showcase' story cards, and borderless scene cards that combine into enormous paintings for some of the most eye-catching work MTG has put out in a long time
Even if the Saheeli Rai has been on the battlefield a few turns and has enough loyalty to survive a Shock immediately, after the Guardian Flickers the Sahelli back onto the battlefield, it will no longer survive the Shock, which makes having access to the red instant at all times vital
Outside of these environments, Urza can still be quite good, as he generates 2 bodies for 4 mana, provides a mana sink with a powerful payoff when needed, and can even turn things like Equipment and Sapphire Medallions into tools to race ahead of your opponents while still employing their inherent value