A feature of the Simple English Propers is that they preserve the mode of the Gregorian original. The original insight here comes from reflecting on the history of simple propers in the old form.
One of the most popular books of the past (and it is probably still popular) is the Rossini propers. These use the same mode regardless of the chant or season. The problem here is that every Sunday sounds pretty much like every other Sunday, and a major feature of the tone and color of the music attached to propers is completely lost. This makes the propers rather tedious in some respects - a perfunctory job we do rather than a special piece designed to elucidate and beautify the text.
Rather than make judgements on which mode the Simple Propers were written in, Adam Bartlett preserved the original, which also allowed the formulaic chants to be adapted to preserve some melodic content as well. You will see this in particular on special days where the introit, communion, or offertory has a special distinctive feature.
To see how this works in practice, consider the example of the Offertory for Pentecost: Confirma Hoc.
Here is the Gregorian original.
Now consider the color, structure, and general mood similarities with the same piece written in the formula for the Simple English Propers.
The idea here is not to replace Gregorian chant. It is to provide a bridge away from a situation where propers have been completely lost and the chant is not being sung, and toward an environment friendly to chant. That transition could take years or even decades, and, in the meantime, beautiful music that is plainsong and melodically fitting with the liturgy is being sung and heard. This is fantastic training for the people and the singers.