Hi, I have a few questions about this rifle.
How balanced is the weight? Is it front heavy or not ?
How reliable is it? Like how durable is the piston and the rifle overall in various conditions(mud,sand,water, etc.) and how well it does in full auto?

Ask the Chinese Army.

Its not too badly balanced.  A bit front heavy, but not overly so.  If you use the bolt on tri rail, and stick a bunch of stuff on it, it can get pretty noticeably nose heavy.  

Reliability is pretty good, assuming you dont have issues with the barrel unscrewing like I did on my Wolf A1 upper.  (Not a design issue/problem in Taiwan, as the barrels are profiled, extensions installed, finished, and chambered here in the US. . . )  
I've heard of folks using them as FA blaster uppers on M16 lowers without too many problems.  Seem to run well.  As far as environmental concerns, reports on the rifle from the Jordanian Army testing indicate it does pretty well in the dust and sand.  And it is in service in Jordan, Kuwait, and the UAE.  

The piston assembly is pretty robust.  And it is amazingly quick to take down, clean, and reassemble.  Some pretty clever engineering there.

Were likely to have much more data in a few months.

The Chinese army doesn't use the T91. Taiwan does.

Users of the T91

Bosnia and Herzegovina: 5,000 rifles
 Haiti: 100 rifles
India: 1,000
 Jordan: 20,000 rifles
Kuwait: 18,000 rifles
Taiwan: 240,000 rifles
 United Arab Emirates: 10,000 rifles

My 16" (with A2 stock and US military cleaning kit, 16oz. optic and loaded 20 round mag) balances right at the front of the magazine well.

There are a few videos out there showing the Wolf A1 running well with a M16 lower. I really doubt that Taiwan (or the other nations that have them) would still be using the T91 if it wouldn't function reliably in automatic.

Thank you. Lots of ignorance out there.

For those missing the joke, it's a reference to the widespread belief that China will begin a program of military expansion, due to the perception of a weak US govt who is friendly to China.  Presumably this would result in enhanced field trials of the T91.

Mine has been pretty reliable with no significant stoppages. Nose heavy depends more on barrel profile than the piston system.

The piston design is pretty elegant and soldier proof. It was obviously designed by real engineers that had good feedback from soldiers.

AJ