Where do I start with McGuffy Reader Third? It was the first of these I've read and it was amazing.
My first impression is the strong sense of the historical time period. The primer is pure 19th century. They assume the children can handle a formidable vocabulary, detailed grammar and pronunciation instructions. Of course, these were to be read with the teacher's assistance.
My second impression is the balance between poetry and prose. The books are structured with stories and poems of increasing difficulty throughout the year. I studied little poetry in my 3rd grade in 1965. It seems to have fallen out of favor in the 20th century.
My third impression is the prevalence of the natural world. Birds, dogs, cats, cows horses, all feature prominently in the little book. Flowers are mentioned many times. In every case the children are assumed to be familiar with all of them. In the 19th century, I'm sure that was generally true.
My last impression is the strong sense of Christian morality. Christianity of the students is assumed and Christian morals are taught from the beginning to the end.
That should be enough for you to decide if you want to use these readers for your own children or school district.