If you're asking if it'd be a good "starting point" for inexperienced players, I'd say no. Always play the game "vanilla" before you do any variants, is how I always feel. In particular, because the game is set around the idea of the normal level runners, and so it will make it easier for the GM to not accidentally overwhelm the players/characters, which is already relatively easy to do in Shadowrun.
As for why it's so popular, I think it's a flavor and storytelling thing rather than anything with balance or challenge. A lot of GMs and players like the "Gangland" style or the more grime-and-grit potential from a Street Scum game. They want the characters to feel like they're working hard to survive, and the lower power level ends up putting more of a focus on the roleplaying. Essentially, it makes the characters feel more "average joe" than the normal process, which presents you as a character who is already skillful or resourceful enough (if inexperienced) to sell their skills in the shadows. Street Scum games are more likely to be based around the characters' personal motivations since they aren't quite good enough to just go find a fixer and make some nuyen, and thus need a different reason to be doing what they're doing.
But as I said, I wouldn't say it's a better "starting level". I'd say it's the opposite, intended for more experienced roleplayers and GMs. The GM in particular needs to have a more involving story than normal to keep the characters motivated. They also need to think more about what they're sending the PCs up against-- Because remember, just because the characters aren't as powerful, doesn't mean all the security guards, Knight Errant patrols, critters, and gang members all got less skilled or violent. Additionally, PCs are likely to be more polarized in skills. This is both due to a lack of resources and because of the story. Non-combatants may entirely lack combat skills instead of them just being tertiary or secondary, while combatants and magicians may be "one trick ponies" with only one area of focus (aspected magicians are more common in Street Scum as well, I feel). This means the GM has to consider each character individually when planning a run, so they don't unintentionally become the weakest link and drag the whole team down.
And, though this is a bit unrelated, for a newcomer, I'd want to offer them the "full Shadowrun experience" and let them make a normal character so they have a bit more to play around with, and thus use and experience when they play. Let them get a taste of the barrens and squalor for sure, but also let them see the high tech wonders and amazing magical intrigue.
