Speaking of house tree person...
During grad school I completed a 600-hour school-based assessment practicum which was based out of the county's department of education. It was good training and I got to attend IEP meetings, talk with teachers, and observe and test children and adolescents at "regular" schools, continuation schools, and programs located within juvenile detention facilities. I was introduced to many reputable instruments -- WISC, WIAT, WRAT, PIC, CDI, PPVT, EVT, Kaufman, Vineland, BASC, Conners, Brown, Gilliam, CARS -- and the Thematic Apperception Test (TAT) and House-Tree-Person (HTP). I never saw the TAT administered or read any reports that used it, but the HTP was part of the standard battery. It occupied a very small part of the final report and the supervising psychologist was appropriately skeptical and didn't give it a lot of attention, but I learned to administer it and basics of interpretation. I did think it was interesting to observe how kids responded differently to the same prompt.
The college has lots of assessments, including the TAT. When covering assessment in class I always bring examples of several instruments and we have some fun with the TAT. I show a few images on the big screen and invite students to mentally project their stories before sharing in small groups whatever they are comfortable. Lively discussions always follow and the image interpretations span the morbid to the hopeful. As a parent, I can't help myself from noticing the kids' drawings when I volunteer in my 6-year old's class. Whose house has a broken window? Does the tree have fruit? What's up with the size of those hands?
Interesting and useful to spring conversation, perhaps reflection, however I don't use them in my own clinical practice and don't recommend them to students.