Not that much of a nerf in general, and arguably a buff - the slow has the same effectiveness as the ministun, generally "stalling" the enemy when they try to close distance against you or try to flee from your attack range. 0.25 seconds of not moving will roughly equate to 0.5 seconds of moving half as fast. The same applies to the attack speed effect. The reason for the change is because Sniper easily countered heroes with channeling spells (Enigma, Bane, etc.) where most heroes have to invest a stunning spell or an item to do so. Bear in mind that Assassinate still mini-stuns, so a prepared Sniper can still cancel out channeled threats - it just means he can't get some lucky headshot and shut them off without much effort.
...Oh, and this.