I'm not a fan of the kicking shield, preferring the heavy bag because it is sturdier and does not rely on having someone else hold my target for me.
No question, the bag rules. The problem is usually logistics: in our setup, a rec center, we have places to hang bags but no bags of our own. So for kicking practice, it's shields or nothing...
I've seen them less recently, which is annoying as they are one of my favourites. They actively prevent injuries. Locking kicks out with no resistance isnt good news.
I use them all the time & have never had injuries with them.
Still, in spite of what I wrote above, I love shields. We just did some kick training with them yesterday, in fact. They convert the smallest space into a real dojang, I think. Bags have their advantages too... practicality rules everything, in the end. I envy people with 5000 square feet of dojang space, a board-holding setup for high kicking breaks, and a dozen heavy bags... but we ain't got 'em! Shields help you eke out the limits on your resources... plus, they give you a very, um,
personal sense of the power of each of your students' kicks.
We have a father and very young son in our school whose English is minimal. The child is almost pathologically shy. But he's getting better. His father is lean to the point of skinny, a bit stoop-shouldered, very hesitant... but in the past month he's suddently come on like gangbusters: My instructor and I were holding the full-body shields for two separate lineups in our class, and when this fellow came up to deliver a series of good-form roundhouse kicks (knee chamber dead-parallel to the floor, all the rest), he practically knocked me onto my back with the force of his kick.
It felt so good!!! A huge amount of progress in a very short time was reflected in those kicks... if it had been a bag, I really wouldn't have had any idea of just how good his kicking has gotten in the very recent past.