SCE on the other hand are nothing but a cheap knockoff of the SM. They are shoehorned into AoS in an attempt to replicate the success of their 40k brethren. Consequently, they enjoy the same treatment (constant updates and attention) as the originals they were modeled off of. In terms of how successful SCE are in terms of this stratagem, I really cannot comment. You AoS players would know better than I. How often do you encounter them on the battlefield? What is their rate of proliferation in AoS? I would suspect that they probably sell pretty well, but it is likely the case of a self-fulfilling prophecy. Give any other faction that much attention and support and their sales numbers will surely rise. I think you make a very valid point about artificiality of SCE proliferation and as you state, it does seem to be very much forced.
I haven't seen or played many AoS battles but when I was round a GW last, even when there was just me, a GW staff member and another player, that player was a SCE player, so I think they are popular. However, they are not as popular as Seraphon, Fyreslayers, Sylvaneth or Kharadron Overlords because the Start Collecting sets for the first three and the Battalion box for the latter are on the Bestsellers page of the website. The starter set is also there but that is the easiest way into the game for all the new players, while the SCE start collecting isn't there, so I'm guessing that there are less existing AoS players who are interested in SCE than Fyreslayers, Kharadron, Seraphon or Sylvaneth, and that most of the SCE fans are the new players who naturally find the easiest way into the game. The Kharadron SC isn't there either yet, but it is quite new still. I imagine when it has been around for longer, it will become one of the Bestsellers.
I also agree here that they are forced, as GW seem to be releasing new named characters for them at every opportunity, while the only character Fyreslayers have is Fjul-Grimnir and he's part of Shadespire anyway. Also he's got very much the same stats as an ordinary Runefather except the Chosen Axes improve his performance. Also Seraphon only have Kroak and Kharadron only have Brokk Grungsson, badass as he is.
I'm not sure if they're a proper "winning" army, cuz despite being fairly overpowered I can't say I've seen any real tournament winning strategy. Basicly they're stupidly difficult to kill and can put out a decent amount of damage with their individual units being self-sufficient. But since they don't have much synergy going on (as if they need it...) they also don't have the option for comboing into something ridiculous which might stop them from just winning everything. There's nothing quite like say a unit of 40 saurus warriors, with a starpriests venom, with a astrolith bearer in a sunclaw host and a scar-veterans Saurian savagery for a ludicrous amount of base-attacks, re-rolls, extra attacks and bonus damage.
I agree with
@Canas here. The SCE seem to be a 'jack of all trades' army where they are powerful but don't have that specialist super-powerful game-winning army build that other armies have. I think they're designed to be an army for new players because they'll forgive new players more easily for making mistakes as they can quickly do something to change it around, while other armies with specialist builds can stomp many enemies but are less forgiving when the player makes a mistake or are against an enemy that is the perfect counter to them.
Probably the main weakness of SCE is their lack of defence against Mortal Wounds, which makes them especially vulnerable to armies like Disciples of Tzeentch and Kharadron Overlords, Sylvaneth being another (I saw a SCE and Sylvaneth alliance take on a pair of Tzeentch armies and get utterly destroyed by the sheer number of Mortal Wounds produced). Armies like Fyreslayers and Daughters of Khaine, meanwhile, have a much greater defence against Mortal Wounds as they have the AoS equivalent of ward saves, which makes them better for taking on armies that spam out mortal wounds than SCE. Daughters of Khaine have an army-wide 6+ Ward Save, while Fyreslayers can have up to 4+ Ward but only their infantry and the Grimwrath benefit from these. Furthermore, these saves counterbalance the less armour these armies have, making them more or less on equal terms with SCE, even though Fyreslayers as an example still have a pretty old book and no warscroll cards (although they did get Allegiance Abilities and relics from the new General's Handbook that has made them stronger in the tournament scene).