Dies to Removal

I'd rather play the exile 3+ toughness enchantment myself if I were looking to substitute Chained in the MD - it hits pretty much the same targets (because how often are 1-2 toughness creatures getting Chained?) and allows you to switch gears to a more controlling role if needed, unlike Ride Down.

I tested 3 Chained in R/w Aggro with 4 MC, 4 Forge, 2 Temple, 10 Mountain and it always seemed like I was either getting the mana I needed and no Chained or getting Chained with either no white mana or no Mountains... The stars only aligned so that I could actually cast Chained in 1 out of about 20 games I tested, so I ended up just dropping it. You might have more luck with this build with a bit more mana which is meant to play a longer game, though.