Kaladin, unlike a coin, does not have an anchor, and even with multiple Lashings only cracked it, not broke it outright. Bigger surface area impacted too. The force of the coin would, I think, theoretically be lessened since Kelsier can't Steelpush the coin against Dalinar without throwing him back - he'd be lighter, especially since the latter has Plate, so in a sense the worst the coin would do is probably turn Dalinar into an anchor of sorts.
And even if regular armor isn't like Shardplate, it would take a leap of logic to suggest that Plate could break so easily. There is a reason Shardplate is used by high-ranking military figures and elite fighters instead of regular armor...
Kelsier would be thrown off-guard by Shards too, I think. For starters, he'd think he could push and pull on them, and when he realizes he can't, it's either take potshots and deal little damage, or get in close and risk getting killed.
That's because the force from a gun isn't constant, while Allomantic force from steelpushing and ironpulling is. And the question of the anchor returns - it's like trying to shoot a hole in a wall with a coin; you'd just get thrown back.