Ok, I'll bite.

I prefer S turns to slips, but that's more about the glider I am flying today. And, when I say S turns, I usually mean two easy turns (back and forth) in order to make the final approach as long as possible. This is usually in a land out situation, dealing with trees on the approach end of the field and only if very, very high (above a full spoiler descent). Basically S turns are the result of a fairly large miscalculation. That would be rare however as it's usually far easier (and safer) to do a circle at 75mph, if truly too high for a safe full spoiler approach. So doing S turns is very rare with the effectiveness of modern spoilers and the performance of even modest performance sailplanes.

All glider turns (at 'almost' all times) must be perfectly coordinated (or any aircraft for that matter). This must be natural for the pilot, effortless in fact. That's the key focus for the pilot during the execution of all turns, correct? Especially so when near that hard thing known as the ground. So, if you are proficient with the basic skill of coordinated flight and follow that key rule, you're fine with a couple S turns. If you don't have that natural skill developed, if your sloppy, your risk is immediately.. That's a far broader problem than S turns.

So S-turns are very safe if you are properly trained and therefore fly the glider properly at all times without stress or difficulty. The only exceptions would be 1) slip landings (which is not very effective (to nil) in a high performance sailplane, in my humble opinion) and therefore I never use slips when flying my personal glider. If attempted (I have tried them a few times), they are very scary as the slip angle must be quite extreme in order to create any meaningful drag. -or- 2) utilizing a slight top rudder configuration whilst thermalling.

Slips work great in a 2-33 or a 126. But even a Grob 103 is fairly poor at slipping. An ASG29 or Lak17a (my experience) gets virtually zero benefit from slipping (my opinion) even in an extreme slip. And with full spoilers, flying relatively slow (65 mph) and at extreme slip angle is not what I want to be doing in any modern sailplane at 200-300 ft (again, my opinion).

Bottom line, turn coordination must be job one for all pilots at all times, from student pilots to advanced pilots. It must be natural and without exception, especially when under stress. When that skill breaks down, the risk increases dramatically at all stages of flight.

So as with all things, and with the question of slips or S-turns (IMO) it depends on the many circumstances involved.