There's a really (really, really) long thread on here somewhere. Might be worth having a bit of a search. To answer your questions briefly. Assuming a 128Gb Fusion:
1)
If you keep less than 128Gb on Fusion Drive, then same speed as SSD. Unlikely, I know, in which case it really depends on how much you access the same stuff. Popular blocks (not files, lower down) are kept on the SSD. So, the stuff you use the most gives you SSD speed. Less popular blocks come up at hard disk speed.
Writes to disk are same speed as SSD for first 4Gb. After that it goes to hard disk. So, if you write less than 4Gb at a time, then same speed as SSD.
2)
The main drawback is that the SSD evangelists will be forever telling you off for buying a 'spinner'.
A fusion drive is only as reliable as a hard disk. If you think hard disks are unreliable, then fusion is a problem. But, hard disks didn't all stop working when solid disks were invented. And backups are a must whatever you choose.