The opinion you quoted is, and I'm being generous here, a minority opinion from those who have played these games in their current state.
Even upon release, while the game was riddled with bugs, the core gameplay was competent, the AI was generally pretty good, and the systems were well-designed (until bugs actively got in the way of them, that is). I remember thinking some of those things at first when I played it at release, but those criticisms went away when I got better at the game. There is a learning curve to the game, moreso for gamers used to being babied by most big games these days, but it isn't like the mechanics are obtuse or impenetrable without a FAQ like many old PC games used to be.
It's worth noting two things here: One, games as ambitious as the STALKER games never have as great individual mechanics as a dedicated shooter or a dedicated action game. Both of those mechanics were actually pretty competent upon release and they have only improved through the care the community has given to these games. Two, the original was similar to Vampire the Masquerade: Bloodlines in that the developers attempted much more with their game then they could pull off, and as such, both games had striking ambitions and achieved much of what they set out to do, but did so in a glitchy or straight-up broken way. After release, the mod community rallied around both of these games and relentlessly improved them into the classics they are today.
Also, the encumbrance thing will not be as big an issue as it is in Bethesda's games. There aren't as many things to loot or pick up in STALKER, which makes sense in the universe, and getting weighed down or filling up all of your inventory slots isn't a huge concern. Guns and suits are the big items that fill those up and unless you NEED to carry around more than six guns (which the game never requires) or several suits, it isn't a big issue. Mainly, the encumbrance system just prevents you from being a walking armory like Gordon Freeman or Duke Nukem is.
