A couple years ago, this may have been accurate, but it's certainly not the case today, now that a book has been published and there are several ridiculous claims that the game is "better" than blackjack.
There may be a few places at remote locations who will leave you alone on Spanish, but in areas where the game is readily available, you're just as likely to get counter-measured and/or backed off SP21 as you are on BJ. More than half of my backoffs in the past couple years have been on SP21. The idea that you can blast away aggressively and get away with it is simply no longer true.
I also don't necesarily agree with the comments that SP21 is lower variance. Certainly the version I typically play (which allows double-double down) is higher variance. The result of several of my sessions comes down to what happened on my max bet redoubles, and I see much wilder session results and bankroll fluctuations with SP21 than I'd ever seen with BJ. I'm not as familiar with the S17 game, but recall hearing that even with it, you double-down more frequently than you do in BJ. That alone would increase your variance.
About the only benefit to learning SP21 is to give you more options. If you live in an area or travel to an area frequently which has a lot of SP21, it might be worth the effort to learn the game. If you're not able to play the game on a regular basis, I'd look for other opportunities.