from a practical standpoint, it has the performance of an SSD. The vast majority of users will notice no difference.
Form a reliability standpoint, this site os on spinning drives. Your bank account is on spinning drives. The files and info Big Brother has on you are all on spinning drives. The iPhoto and iTunes libraries that you'll be hanging off your iMac because its SSD is too small will also be a spinner, unless you pony up more big bucks for en external SSD. Yes, they will eventually fail, but they're usually nice enough to let us know before they do, unlike SSDs. I back everything up and but Applecare (things break).
I don't know about you, but I've got pretty close to 2TB of data. This is about to explode, as I'm now playing with 4k video. So, I'd have to buy the 1TB internal plus a 1TB SSD for my libraries, if I wanted to avoid spinners. Shucks, since my data is growing, make that 2x1TB of external drives, to have the same capacity and roughly the same noticeable performance as a 3TB Fusion. Actually, the reads on my external drives will be significantly lower. So, if you really didn't want any spinners, prepare to cough up ($600 for the Apple 1TB upgrade, plus about $1,000 for two external SSDs. An extra $1,600 for the same capacity and roughly the same performance. Internal writes will be a bit faster, but all the external work will be slower.
I say again, It will cost you about $1,600 to meet same capacity and roughly the same noticeable performance as a 3TB Fusion drive, using SSDs. And still, you'll be backing all that data up on spinners. Now, if you chose to save money by directly attaching spinners for your external libraries, you've just robbed Peter to save Paul. Transferring photos and videos back & forth will be painfully slow. So, I guess the question is - Are you sure you'll only need 512G or 1TB of data, several years from now? If the answer is yes, then SSD away. if the answer is no, consider a Fusion drive.
A Seagate went bad on my iMac, about 5yrs ago. It barked at me for months telling me it was;t happy, before it pooped. I kept backing stuff up until I brought it into Apple (Part of a recall). Problem solved.