Not all, but most are very bad.
...
By the way, I am satisfied with 3 of my 13 Fnirsi products, I got what I expected, but the rest are... 
Yeah, I'm not quite convinced as to your veracity here.
Why would you keep buying them if they don't work properly, or are you just another one that refuses to read specifications, then judges a product on pre-conceived notions rather than on published facts? Like your assessment of the 2C53P - "lacks many features". Exactly what features did they promise that they did not deliver"
I have tested the following items personally, and in all cases
I READ THE F"N MANUAL before I got them, so was not surprised out of my mind when they only came with the claimed functionality.

2C53P: Met all specifications and indeed exceeded some. All claimed functions were delivered. I did not find any "bugs" - maybe, if they are not of the type "It didn't work they way I think it should" you could provide a list of these "bugs".
DSO-152: Met all specifications. All claimed functions were delivered.
DMC-100: Met all specifications. All claimed functions were delivered.
DMT-99: Met all specifications. All claimed functions were delivered. Only thing that bothered me was the non-standard jack spacing.
HS-01: Met all specifications. All claimed functions were delivered. Impressive for the price.
HS-02: Met all specifications. All claimed functions were delivered. Personally prefer the HS-01.
DPS-150: Met all specifications. All claimed functions were delivered. Impressive noise figures and response to load changes. Again, non standard jack spacing.
LCR-ST1: Met all specifications. All claimed functions were delivered. When compared to others in it's price range, it comes out ahead.
LCR-P1: Initial release had several issues: Most of these have been addressed via firmware updates. Still leaves something to be desired.
FNB58: Met all specifications. All claimed functions were delivered. Impressive set of features for the price.
FNB48S: Met all specifications. All claimed functions were delivered. Spend a bit more and get the FNB58.