I'm not a fan but for bulky items they are adequate. When you add weight then I think the foppy becomes a problem especially if things start clinking, banging and in general rattling. I have seen a green horse have a blowup over a popping lid on wood panniers. Silverware in a metal pot could trigger an all day event.
I have mannied on a saddle set up with 2" d-rings behind the cantle. With a little more effort, a set of canvas panniers can be made to be saddle handy.
Nomatter what you chose I would still use a lash cinch to minimize the flop in your journey. If you are starting out, pick up the book on packing from Back. It's entertaining with a lot of make do attached. Suprisingly the last copy I picked up was in an Exxon station in Dillon,Mt.