Tameshigiri is ok. As long as the sword goes through, nothing bad happens. That is similar to breaking blocks with your fist. As long as the thing breaks, the impact to your body is minimal. But if you don't hit quite hard enough, the resulting impact can do bad things. The latter is what can happen with partner training, weapon on weapon.
I had a few swords very similar to this at one time. A friend of mine at the time who was involved in some iaido was handling one of them. He managed to break the top third cleanly off attempting a cut. Very lucky it didn't hit anyone. Not to mention, you can count on the fittings to loosen with use and fail after a very short amount of regular practice. Anyway, there's not much value in sparring with sharps. The restraint required to make it feasible eliminates any benefit from risk exposure and will likely create bad habits. Using fukuro shinai (allowing you to move at a realistic speed with realistic intent) will produce a better mindset anyway, if you're looking to experience the tempo of a real fight.
That actually makes sense. When you swing a sword into a cut, the sweet spot for maximum stress is around that point, 1/3d from the point. If you connect with that, you have the least amount of vibration, but a maximum amount of localized strain. With the sword in a stationary position, the sweet spot is just below 1/2 of the blade. If you strike there when the blade is used for blocking, that is where you get the highest chances of breaking the blade clean through.
