The biggest problem as I stated in the video that Russ did; is tha the teflon o-rings would become uncompressed from the side air plug. This made leaks notorious and at the time, MILSIG was just a small company with half of the company being people that came over from Airsoft.
As the market has grown and as both the head of the manufacturing and the head of the company brand have taken a more serious note to the market and the community they have vastly improved their products.
The HEAT core is a major step up and eliminates a lot of the problems from the past. No more "air restrictor" style velocity adjustment. A proper pressure based velocity adjustment. Easy to strip down and clean.
I used my blow back MILSIG for 2 1/2 years up to the Para Pro and have not really had any problems with it. Having the right information, the community and the likes ensured my gun ran on the field.
I remember when I first started playing (and it being my first marker before I even finished working on my pump) people gave me all sorts of comments about it.
Right kind of lube, some care, and it'l go well.
The first gen was a mess, the tech support just wasn't there as strong as it is now, and the company mentality was not set in stone.