Ike wields a heavy broadsword and Krom carries the Falchion. Krom wouldn't be nearly as slow (or powerful) as Ike.
Sealed Sword Falchion (Krom's sword) and Falchion (Marth's sword) are completely different blades. Just look at the size of Sealed Sword Falchion:
FE9 and FE10 were also the nadir in terms of sales for the FE series in Japan, with the only other game selling below 200K in Japan in the FE series being FE5 (which was a Super Nintendo game released in late 1999). Heck, FE9 and FE10 COMBINED sold less than FE6.