I don't take Ally's word about things for Snake. He's a simple character with simple options, and I don't think he's that good.
For all of you disregarding . . . I think it was shmot's conclusion about grenades . . . you're the foolish ones. You SHOULDN'T be getting hit by grenades often at all. A grenade gets pulled, you know that in three seconds, you need to NOT be vulnerable in it's range. Most people do this by OVER-CAMPING, or they approach incorrectly, and hence often hit grenades.
I typically do not get hit by grenades, and often, if I do get hit by grenades, it's because I made a mistake.
As for punishing Snake's ftilt . . . it has implied safety. Ftilt1 might have lag, but it can be followed by ftilt2. If we disregard reaction time, and you buffered a grab as soon as ftilt1 hit your shield, and ftilt2 was used at the first possible moment, I believe characters with a frame 6 grab (the fastest and most common) would grab it, but trade the hit (grab armor). However, if Snake is hitting you with an ftilt, he shouldn't be in grab range.
Because of this, staying in shield is a reasonably good idea. Just don't freeze up so long that you reset the trap multiple times.
OH! I kept forgetting to mention this. Ftilt 2 doesn't have enough lag for ICs to Dash Grab (they can punish, just not with grab). A running or power shield obviously helps, but it's reasonably safe to do to pressure ICs shield.