Developmentally, Risto was never done well by the Sabres. They rushed him through which was a hallmark how late-stage Regier damaged the future and helped lay the groundwork for the last decade of suck. He was not ready mentally for the game when he arrived and that didn't matter - they saw "big, strong, physical guy" as he is always described and plugged him into a terrible lineup and just expected him to swim. He wasn't a good defender in the AHL prior to getting the call, never worked out the problems in his reads and puck movement.
Then after the horrors of Mr. Gardening Gloves and Mr. Competeez, err Rolston and Nolan as his HC, he got Commander Stretch Pass as his HC and the team tried to play Bylsma's style for two years. Yes, he was struggling and almost without fail, we could isolate on the defense during the breakout and you wouldn't see a forward in frame, let alone close enough to get a quick breakout.
Then came the Pokemon equivalent of Rolston's final form: Phil Housley. One of their departed players talked about having over 60 plays to break out. His system was overly complicated and placed demands on the defensemen that were now different that Abysmals' but weren't actually any better. At this point, Risto is one of many defensemen who are making the wrong decision at the wrong time regularly. McCabe and Bogosian (on the rare occasions he wasn't too hurt to go hunting, err play hockey) would also make terrible positioning and outlet decisions on the regular.
By the time Krueger came along, and he's another f***ing gem of a compressed turd as a HC, Risto's cement is basically set. There were two brief interludes in all of this were Risto's fancy stats bounced up to at least a break-even point -- when they were forced to play him with Lawrence Pilut. Pilut, now chased off to the KHL (Thanks Botts, you big-headed twit), is a capable, mobile puck mover. Risto could do whatever and often Pilut would be able to exit the zone. Yes, Risto's dubious positioning and tendencies to just drift toward the puck like he's in mites are still there, but when he does get possession and can go d-to-d, his partner is able to work the puck up the ice.
Risto needs a strict role defensively - go protect the net front and chip to your partner or a low forward who is close enough to help with the gap (something they sometimes do now with Granato as HC but haven't done in the last decade since Ruff left). He needs the help because he doesn't process well and his panic threshold is not developed and likely never will. Offensively, just let him rumble in the offensive zone.
The other option that some vocal minority on the Sabres board proposed for a time and got a little bit of play when he was used as a net-front PP guy is just convert him to a winger. Let him worry about playing the body and getting to the net, try to be responsible defensively, but it's not nearly as bad having him fumble away pucks or be out of position if he's on the wing. Then it's just a big Finnish version of Jeff Skinner.