For my recent trip, I managed to snag a last minute Senzantei room at Iwanoyu(after reading) @KI-NRT post. I had been placed on waitlist which cleared about a week before my indicated period of stay. I had make some contingencies in my plans so that I could jump on this if it cleared. Here is another thing I have discovered, last minutes rooms for sengotei and senjutei do appear close in on ikyu(5-7 days out) due to cancellations but not senzantei as there is already a long waiting list for those rooms. Here are my impressions for Iwanoyu and Yunohanaso in Nasushiobara.

Iwanoyu: Senzantei rooms are really well kept and I appreciate the unlimited private baths(no bookings required) that you get with this room type. The cave bath was just alright for me. I very much preferred the private baths. The futon is also one of the most comfortable I have ever slept on and the staff are amazing. The property is so huge that you seldom bump into other guests and the food quality/quantity is fantastic. I will return definitely for senzantei benefits as the price point is excellent for what you get.
Edit:Wifi is also spotty in the Senzantei rooms but it is fine in the common areas. I relied on cellular mostly while in the rooms.

Yunohanaso: Stayed in the recently renovated(March 2025) Owari room. It is a good solid property and the food quality/presentation is great. Lounge in the lobby has complimentary alcohol and self made filter coffee in the morning which was a nice experience while enjoying the view. However, there was nothing truly outstanding or unique that might prompt me to return unless I need to be in the area. One small nitpick that I have was that in-room onsen was a little too cold to my liking.