This; this, right here, is a problem. All pieces of ice before have established that:
-Sentries wreck you
-Code Gates tax you
-Barriers stop you
And there's not much deviation from that. The important upshot of this is that, strategically, it's generally safe to run as long as you have a scalable killer, you can run safely; the worst thing that can happen to you is losing 3 credits (tollbooth) or taking 2 damage (wall of thorns). Nastier code gates and barriers exist (viktors, heimdalls) but they usually end up costing you clicks, enigma-style. Hence, the popular wisdom: "never run on your last click".
If you run, turn 1 click 1, the worst thing that can happen to you is losing your hand to komainu and then jacking out. If you install faerie and run, the worst thing that can happen to you is losing clicks (bioroids, enigma, hourglass) or credits (datapike, popup, various tracers) or taking a minor amount of net damage (yagura, wall of thorns).
Shiro kind of breaks this rule by making you access snares, but if you're running you're ready to access snares anyways. Inazuma also breaks this by turning archer into a str 5 code gate, but Inazuma's positional restriction make it much easier to predict. If you see two pieces of unrezzed ice, or a nasty ice behind an unrezzed ice, you should get a decoder, just in case.
Merlin has a non-negligible chance of flat out KILLING YOU if you run turn 1 click 1. A corporation with 3 Merlins in their opening hand simply has to install ice, click up 1 credit and wait for the runner to make their fatal run. Face-checking just became a lot more dangerous. Running turn 1 click 1 against anyone is no longer a correct play; drawing up to six is required until they can't rez a Merlin or you're confident they don't have multiple Merlins in hand.
Now, you might say the odds of this happening are small. And you're right. The odds of getting all 3 Merlins in your 6-card opener, in a 49-card deck is ~0.1%. One game in one thousand. A freak occurrence. (try it here if you don't believe me: http://stattrek.com/online-calculator/hypergeometric.aspx)
That's not what makes Merlin dangerous.
What makes it dangerous is that it's not a sentry.
Imagine if Neural Katana had two cousins, Neural Broadsword (a barrier) and Neural Trident (a code gate). Suddenly, you can't facecheck ice without fear of taking punishingly large amounts of net damage. Suddenly you need a Faerie, Corroder and Gordian Blade to run anywhere.
Merlin is a Super-Neural-Trident; If the corp has unknown cards in hand (which they usually do) and 6 credits (which they sometimes do) then every face-down piece of ice is a non-yoggable lethal code gate.
But it's just net damage, right? It punishes face-checking, but just like Komainu you can pick yourself back up afterwards if you had enough cards in hand to survive it, right?
Well, no. It gets worse.
Merlin is a program-trashing code gate. It's not a destroyer, either, so sharpshooter doesn't help.
God help us if they print a barrier grail, which they probably will because this looks like a cycle:
-The typeless, "mythic" grail)
-The Sentry grail
-The Code Gate grail
-The Barrier grail???
This thing, and the likely-to-be-printed barrier grail are going to be major sticking points in the future; they're going to be the things you need to plan for and you need to constantly be aware of, like account siphon is for the corp. Back-breaking if it hits you unprepared. They're neutral, so they'll be in every deck; at least they cost some influence.
I don't want to be the alarmist "broken-the-sky-is-falling-second-coming-of-caprice" guy, but Merlin breaks a lot of unwritten rules. It's going to cause some upsets.