I have a 28 button Jeffries and on the basis of that plus playing and observing other like models I would like to add a couple of comments. As far as I can tell, the 26 and 28 button vintage concertinas preceded the development of the 30 button models. You shouldn't always assume that a 26 or 28 button concertina is in some way inferior to the more buttoned models - at least not for a Jeffries. Mine is top quality and I'm familiar with similar quality 26 button models owned by others.
Make no mistake, there are some drawbacks and accidental row note position is one place were a number of changes are evident. For instance, on my 28 button Jeffries, the low press A is on the last (or bottom) button on the outside row and other note combinations are similarly shifted over one place by the absence of that 5th button on the outside row. This does change fingering a bit. The right side is less affected since in the Jeffries note layout the first and second buttons on the outside row are just inversions of each other. So the note assignments of the other button notes in that row are unaffected.
Overall, I am very pleased with this instrument. By virtue of the fewer buttons, it is lighter than 30 button models. One last and important difference is that these models (less than 30 buttons) are frequently less in demand and may be acquired for more attractive prices. So, to paraphrase an old TV ad, "Try it. You'll like it!"