Zastava receivers are 1.5mm thick instead of the 1mm AKM spec. The buldged trunnions are left over from when Yugoslavian guns were set up for grenade launching. There was a period when Zastava was cutting corners and Century Arms was F-ing up the imports. Which made for

rifles. But Zastava USA still has considerably better QC than Century let alone any of the

show US companies.
Yes, they are heavier than stamped AK's. But still lighter than milled AK's.
Yuga pattern rifles (both SKS and AK derivatives actually) use different furniture than the original Russian specs call for. Yugoslavia wasn't under the same pressure to standardize parts as the rest of the Soviet bloc. That history is rather well documented elsewhere and not just about small arms design.
That said, people that were in combat in Yugoslavia and found the chance to pick up Romanian AK's apparently did so because they were lighter than Yugo rifles.
WASR-10's can be good enough, cheap, and reliable. They're clearly not as nice as East Germany, Polish, Bulgarian, Hungarian, or Russian AK builds, or modern Polish stuff, but still, generally, considerably better than any US made complete rifles.
KUSA rifles are known to have had a variety of issues over the life of their company. Now that their gone, you're going to have a hard time getting things fixed in case they do fail.
Do your research on known failure modes for the 103's. Get headspace gauges and keep an eye on the headspace.