I have mixed feelings about sidepulls, all bitless bridles really, in general.
I think they have their place (no experience with them for initial training like Kitten_val mentioned but I'll take her word for it) as a tool on the way to an end.
My biggest...caveat, I guess, is that I do not think they should be used exclusively, longterm.
My mare has melanomas in her mouth that make any sort of jointed bit totally out for her. When I was first discovering this about her, I started riding her bitless (in an "Indian Hackmore") exclusively.
At first she was great but after about 6 months, there started to be little issues popping up here and there. She's already a bolt-y type when excited and she quickly figured out that I had no "backup" with a bitless bridle. I tried and tried to reschool her in the bitless bridle but she just stayed hard no matter what I did.
To make a long story short, it was a long road back (by way of a low ported pelham where I HAD major backup if I needed it) and it's only now that we're able to go back to any sort of bitless for normal riding.
Now I switch back and forth from a low ported snaffle to bitless every other ride or so and she stays so much lighter.
Currently my favorite "bridle" is my self manufactured riding halter (the reins attach to the rings below the nose knots and I use weighted reins):
There's enough "play" to it that she gets a pre-warning, then a warning, and thennnnn I get in her face if she doesn't respond. I think that's been the key for us - the bitless bridles we've used didn't have enough pre-warning, they were constantly either totally off or REALLY on.
Of course, I don't ride her anywhere I'd need to contact her face and stay there (not my favorite but when she gets to excited there's really not much of another choice) in that bitless thing because of her history. If I tried to do that, she'd just barrel through and be like "seeya guyz!" like she used to, unfortunately.
I see no problem with the sidepull you posted, but just be careful. Some horses can be ridden exclusively in bitless shindigs but the majority just can't function well like that.