Jim, I won't pretend to know everything about these, but they are not a bad design in the right application. There were a few that were put on geared engines as delivered. An Ed Hines Lbr. 70-3 Shay comes to mind. There was also a big Heisler back east that ran with one right up to the end of her career.
One more thing that I'll add here. These type of devices are called direct contact or just contact heaters where the exhaust steam comes directly in contact with the water from the tender. Also referred to as an open type. They had something like 236 parts to them.
The other type is the closed type & these are the ones where you see the big tube bundle (beetle brow) up ahead of the stack. There were a very few SP Mikados that got these...Texas & Pacific put them on their big 2-10-4s. Both types required maintenance & cleaning.