High inflation of STEEM one of the most significant risk that's being factored into this 7-day lock-in period.

as pointed out by @hisnameisolllie in his article: STEEM Dilution Rate - MUCH higher than you might think… at current level the dilution rate of STEEM is nearly 1% per day. That means the STEEMs you get 7days later is worth ~7% less compared to instant conversion.