UKC

The question I had is how safe is continuous climbing when multipitching? Obviously the safest way to multipitch is to stop at every pitch and belay your partner.

But I see a lot of advanced climbers on the internet just tie into an end of the rope, while person A sets all the quickdraws and person B collects them. 

So what are the dangers?

What happens when falling?

When should you continuous climb?

Like normal lead climbing plus the risk of getting pulled off the wall by a falling partner. Also recovering/retreating from a fall is likely to be more complicated.

Much like usual but if you're the top climber being pulled by the lower one the slam when the rope pulls tight will be horrible. If you're the lower one you fall as iff you're on lead until the higher climber slams into the gear.

When the extra risk in falling is outweighed by the extra safety increasing speed delivers. It's most commonly done (outside of big wall record attempts) on relatively easy broken ground with lots of natural runners, ridge scrambles and that type of terrain.

jk

Not exactly a danger, but person A is sooner or later going to run out of material.

Exactly. You'll likely have to run it out to make it worthwhile. Plus all the other things mentioned above.

To answer the OP, overall I'd say substantially more risky than traditional pitching and a lot more scary. Only for when you really feel the need to move faster and can accept this extra risk.

Indeed even pros will only simul-climb in the easy sections of a big wall, for the harder sections they'll short fix...

Weren't a couple of guys killed in Yosemite last year while doing this.

Far better than classical simul-climbing or short fixing is simul climbing whilst protecting the second with a rope capture device such as the kong duck. The device is attached to a piece of multi-directional gear (a bolt or bomber cam/thread) with a locking carabiner and fixed on the rope to allow feeding in the direction of the leader meaning it would lock in the result of a fall by the second. 

With 2-3 of these devices it is possible to climb 100-150m pitches even when only using a 50m rope, provided you have a sufficient amount of gear. 

Example use: Last autumn my girlfriend and I arrived in Verdon after driving through the night and had a well deserved snooze. By the time we woke up there was only a few hours of light left but we still wanted to get a route done so we resolved to simul-climb something steady. We used the described system to climb a 6 pitch 6b (150m) in just over 1.5hrs including a midpoint belay to exchange gear and swing leads. At an average of 15 minutes per pitch this saved around 10-15 minutes per pitch compared to standard multi pitching without any tricks. My girlfriend took one fall due to a slip whilst seconding, but as leader there was no impact on my side of the rope. 

In my opinion this is the superior system for moving fast whilst staying safe. Personally I don't go on a multipitch without a lightweight ascender like a microtraxion now so am always ready to protect the system if necessary (plus it's great for if you need to do a rescue!). 

The only point where you need to be cautious is when using long lengths of skinny ropes - your second will bounce some way if they slip off because all of the rope is out. However, we've been happily using our triple rated 8,6 Canary with this system and the stretch is very manageable - maybe 3-4m so you need to kick away from the wall a bit if directly over a ledge but other than that you're good. A thicker rope negates this so if using without a long walk in where weight is crucial then a thicker rope makes life easier in that respect. 

Hope this helps!

Yeah a couple of really experienced climbers took a fall last year, can't remember if they were short fixing or simul-climbing at the time? 

Also Quin Brett took a bad fall whilst short fixing last year, she's alive but now in a wheelchair.

In short (no pun intended) both short fixing and simul-climbing ramp up the danger level pretty dramatically, and you ought to be a really experienced climber before you start thinking about either giving either of these a go... 

This reminds me of Tom Patey's account of his conversation with Arne Randers Heen when moving together in a three with very little gear on the Romsdalshorn:
"Why are we roped together?"
"One of us might fall!"
"That's what I'm worried about..."

They were short fixing with one cam (no.2 friend size from what I recall) between them on 5.7 terrain. It would have been their collective 100th ascent of the route or something along those lines. 

For the OP: I'm all for safe measures to speed up on easier terrain (I consider my method mentioned previously to be one of these) but removing gear from the system is a really good way to die or get paralyzed. Consider changes to your safety system with extreme caution. 

When you ask, how safe, I’m not sure this has an answer. Not as safe as single sports climbing, but probably safer than alpine or ice.

The dangers come from various things

You are moving fast and hence thinking fast and might slip etc.

You are probably placing less gear

The microtrax etc. (the only approach I use unless almost scrambling) might pull its anchor, possibly from being jiggled around a lot

If someone falls, they will be a long way away, hence more stretch but also very difficult to reach. Or communicate with

You probably only have one rope, so evacuation more difficult.

You can not lower the second to a ledge if they are hurt.

You will not be initially be mounting a rescue from a belay

Alongside full blown simul is the possibly more common practice of taking a trax with one then deploying it for the odd pitch. For example to remove those annoying 30m scrambles between one pitch and another when you are not sure you have enough rope left, or drag is becoming an issue (you can yard rope through a trax and it will not fall back down), and when the moves off the lower belay will be easy for the second.