Teacher evaluations are dubious for a couple reasons. The first is the fact that principals, in my district at least, evaluate based on 3 observations in the classroom. These observations do not really provide enough evidence of teaching or not-teaching, unless the principal is predisposed to evaluate you in a certain way--positively or negatively. In that case, they can point to just about anything they want, and leave out anything want, to make the evaluation appear any way they want.
The second problem, related to the first, is the fact that the principal does this alone. There is no other person there, observing the same thing, to compare notes with. Perception is not always reality, and 2 people looking at something may just see things very differently.
Because of the way teacher evaluations are performed, teachers, unless they are on good personal terms with the principal, must live in fear of the baseless negative evaluation for which there is no recourse because it is the principal's word over the teacher's. As we know, management tends to win these battles.
I have many, many letters of appreciation from parents of students, and have had master teachers and principals tell me I am a fantastic teacher. I have been asked--by my current principal!--to be math leader (I said no), disaster coordinator (I am currently), tech leader (was until we got a new teacher who used to be tech guy for his district--he knows way more than me!), and am in charge of whole-school outings. Yet, because I push back, my principal gives me negative evaluations. This year is the first year I have ever received a negative evaluation. My teaching has not changed; indeed I feel I have gotten better having been left in the same grade for 4 years in a row (something I have written about before).
I would like to see teacher evaluations done by fellow teachers in conjunction with principals, and I would like the written notes of all evaluators to be included, with equal weight, in any final evaluation. This would be more informative and much more fair than the way it is done now.
For all you "unions protect the bad teachers" folks out there, the union can do nothing about a principal hell bent on firing a teacher; we teachers don't have any mechanism to expose the nonsense; union negotiations see to that (we lose, always).