You should remember that the games Doyle describes in Super System were not the 100xbb max buy-in games that you're probably playing in right now. His games are structured a little differently, so let's talk about how to convert them to a two blind game.
A three blind game of 25-50-100 he prefers to sit with 20,000. That's 200 times the big blind, or 114x the sum of the blinds each round. Compare that with a modern game where the buy-in is 100 times the big blind or 67 times the sum of the blinds. In short, these were much deeper games.
In that game, a pot sized raise of $400 and a call (assume the blinds all fold) will leave you with a 1,200 pot (1175 but who is counting). That's an SPR over 16. Suited connectors play really well with these high SPRs. And, the games were 9 handed and a lot more passive. You might get several callers. So, your SPR would still be fairly high, and you would have plenty of people left to pay you off. And these were minimum buy-ins, he talks about buying in higher and wanting to cover the biggest stack at the table. These were deep games. There was also a lot less three betting and four betting and so on.
In a modern game, you raise to 3.5xbb, get called, and the pot is 8.5xbb on the flop. Now the SPR is around 11. If you get more callers, you benefit by having a higher chance that someone can pay you off, but the SPR falls dramatically. If one person and both blinds call (4 opponents), the SPR is now under 7. Suited connectors don't play well with medium and low SPRs because they take time to develop. You also can end up getting 3-bet squeezed and having to fold them.
Suited connectors are fine, they are good for keeping your range balanced and making it so that you really can hit most flops. But, you won't lose much money if you don't play them 6-max and 100xbb deep at the lower limits. You don't gain a whole lot when you do. But, as you and your opponents both get deeper, they go way up in value. If you're 200xbb deep and your opponent is as well, you should be looking to play a lot of pots in position with those suited connectors.
They do have a lot of value. And Doyle isn't wrong about them, but you have to put them into the context of the environment.