SoS is a fantastic scoring system. I'm currently working on a video explaining why, but here's a (very brief) rundown:
All WTA systems encourage a Solo as the #1 priority, which is how Diplomacy should be. However, the difference in how they reward draws (if people are paying attention) makes a very big difference to the game.
Take DSS, or Rulebook Scoring. The #1 incentive is to win, obviously, but the incentive to get more points in a draw is to whittle the draw, force the elimination of other players even if it's not you who gets their centers.
Most good players will tell you that in order to solo, you should keep players on the other side of the board as balanced as possible, or if that's impossible at least try to keep as many players alive as possible so that there's more conflict going on there and it's less likely they'll be able to unite against you. The more players there are, the more likely it is that two of them have some sort of grudge with one another, and the harder a time they'll have coordinating. If there's one very strong player on the other side of the board, chances are they'll be able to stalemate you.
DSS's incentive system works very much against this. You have to think "Do I want to aim for a solo here, or do I want to try to increase my score if there's a draw?", which is not ideal. Of course, you can try to position yourself so that you can back off the line if you do get stalemated and allow players to be eliminated, but (especially if you let a lot of players survive to this point) it's very possible that they don't trust you enough to stab one another, and you're stuck with a lousy 5 or 6 way draw. An ideal scoring system allows you to work towards a solo while simultaneously working towards increasing your draw score - you shouldn't need to have to give up the shot at a good draw to go for the solo, and you shouldn't need to have to give up your solo chances to go for a draw.
SoS solves this problem. In Sum of Squares, you will always gain points from gaining Supply Centers, but you'll lose points if your largest opponents are gaining Supply Centers at the same time. The ideal distribution for centers in a SoS draw is for you to be the largest power and for every other power to still be alive and have their centers split evenly. That's also the best thing to aim for when trying to solo - so in SoS, the draw/solo scoring system allows you to work towards both a solo and a good draw at the same time, instead of forcing you to pick which you'll attempt to achieve.
One point people make against SoS is that small powers holding a stalemate line are often minimally rewarded for their survival, but these people miss a massive upside of SoS - since small powers gain very little from being part of a draw when they're on 1 or 2 centers, they have a massive incentive to throw the game if someone annoys them, and so any threat they make has to be given high credibility. A small power can demand a few extra centers from a larger power holding the line, and threaten to throw if they don't agree - and the larger power has to agree to it. I think people often miss this in SoS and it's a strategy more people should use.
Another complaint is that Draw Whittling is an integral part of Diplomacy and should be kept. I disagree - if you're whittling the draw by letting someone else take the centers, why should you be benefitting from that? SoS gives a situation where it's in the interests of the player(s) actually taking the centers to whittle, but if you aren't benefitting from it on the board then you don't benefit from it in the scoring. It gives smaller powers more options for survival, and stops large players letting other large players take a bunch of SCs so that their score is increased.
DSS is very good for players who like working towards a 3-way draw and are not good at achieving solos. I know this because I'm one of those players - but I'm trying to get better at soloing, and in DSS that requires giving up the ability to get a good draw. SoS is good for players who are good at working towards solos, because it essentially rewards you in accordance to how close you got to soloing.
One final remark - DSS is almost certainly better for players who use the 'Let's go for a 2-way draw' strategy to solo their games, because in SoS there is quite a large incentive to draw when you're close to the two way draw, but not quite there - you get nearly the same amount of points. However, alliance play is still very rewarding and although that ability is diminished, it's still very possible to convince someone to go for it (or to position for a stab slightly earlier).