I would argue that they are equally powerful, there are just less decks to host JtMS in modern. When Jace lands in UWx modern and survives, the game is over quite quickly, which is also the case in Legacy. Miracles, the equivalent UW deck of Legacy plays JtmS in a similar way as modern UW control.
In legacy we also have Sultai decks that feature him, and also UWx stoneblade that has the potential to run him, strategies which are under-represented in modern for reasons unrelated to JtMS.
In legacy we also have Sultai decks that feature him, and also UWx stoneblade that has the potential to run him, strategies which are under-represented in modern for reasons unrelated to JtMS.
UB Faeries (15-6-0)
UWR Control (10-5-1)/Kiki Control/Midrange/Harbinger
UBR Cruel Control (6-4-0)/Grixis Control/Delver/Blue Jund
UWB Control/Mentor
UW Miracles/Control (currently active, 14-2-0)
BW Eldrazi & Taxes
RW Burn (9-1-0)
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