Janet
Best tankbag option is baglux, but unless u can get them cheap off ebay, rather dear initially. But they are superb, the cover straps firmly to the tank( no magnets) and the bag securely clips to the cover.
On my speed triple I often used a oxford mini tank bag , ( magnets) which never moved. I also have done a long tour on my fazer with just an oxford magnetic tank bag with no issues.So long as they are fitted properly they are no problem.
If anything , people have had issues with tailpacks, which are potentially lethal if the bag slips round.This actually happened to dek at high speed, and he fortunately lived to tell the tale.
I have used oxford soft panniers on my fz-1 but the fixed grabrails make securing them a bit awkward.
The other option is a ventura soft bag system. I like this, but again there is initial expense to buy the rails etc, but once fitted is a doddle to take the bag on and off. Its 100% secure at speed too. 150+ on german autobahns no problem.