After a year I have the following problem, pieces of plaster (1+ inch
deep, 4"x5" on the surface) from vertical walls occasionally (every 4-5
months) fall off. The problem according to a contractor is the cement.
I think that's without doubt as the plaster falls of with a small piece
of cement always ( about 1/8" ) .

The reason for this was attributed to bad cement mix: The original
fault I was told was the addition of too much water to the mix. does
that sound plausible? provable? The gravel seems to be also a bit too
oblong, slits rather than rounded, but of medium/fine size.

The remedy is quite expensive, injections into the cement. But only
local treatment. Given the rather rare occurrences, and since I repaint
every 2 years maximum, does it really worth it? ( total house surface
1600sqf)

Other solutions
- total replastering ? ( with global cement treatment)
- glass fiber cloth ? ( on existing plaster, without cement
treatment)
- covering with prefabricated plaster on metallic trellis base, on
top of existing regular plaster.

Your ideas?

I could also pursue legal remedies but... am not sure what is best.
it's a bit of gutting experience.