Tulsamal pretty much nailed it, IMHO.
I have personal experience w/6 different STG's from an early Gen2 to a late Gen4, appx 10K+ rds cumulative total.
One had to be returned to MSAR for extraction problems (promptly fixed & returned), I've had several of the old-style extractors break (new-style extractor is GTG) and broken two bolt sleeves to date.
All other problems are related to new rifles that haven't worn-in yet (mine have all come tight from the factory).
All my STG's are 100% w/brass-cased ammo but reliability w/steel-cased varies from rifle to rifle (just like w/AR's) w/at least 100rds of full-power 5.56 required before steel-cased ammo starts functioning reliably (YMMV).
I've considered other platforms but keep coming back to the size/handling advantages of the STG (16" ballistics in an SBR-sized package w/o the paperwork). Nothing else handles as well for me, even after running them through a couple of carbine classes (where my best friend [cop] fell in love w/the STG so I ended up giving him the one he ran the class with).
If you buy now you're taking a small risk of getting a "bad" one but w/upwards of 15,000 STG's/E4's out there I think the risk is very small (again, YMMV).
HTH...
Tomac
Some of the STG's in my gunsafe w/various optic options:
Best friend and I at carbine class:
Best friend doing run & gun w/moving target & no-shoots: