Yes hannu I think you are right. To produce any satisfactory small reaction turbine you have the effects of scale working against you.
The engineering needs to be a high standard, bearing friction, clearances surface finish and the problems of blocking the small dimensions are all a big factor.
Impulse turbines are not really low head devices and are not used as such in large sizes but they can still be efficient at low head if you can build a low speed alternator. They seem much more practical in the low flow region ( 100 gpm and below).
For anything above 6" diameter then the reaction turbine should work out the better option if you can build it carefully.
Flux
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