I think Raonic played good in Halle two years ago, when he lost to Federer in a 3rd set tiebreak or something like that but has done nothing on grass since then. My impression is that grass is the least favorite surface of Raonic and Isner as their weaknesses return, movement and the inability to consistently handle low balls are exaggerated, so they basically score no breaks at all. Also they hold their own serve nearly everytime anyway, so the surface can't help much in that regard and might even lessen the effectiveness of their kick as a second serve. Karlovic is somewhat different as he goes for rather flat second serves more often, plays S&V, has a solid backhand slice and is fairly good at low volleys.
In terms of probability to get to the quarter finals my ranking of the 5 to 8 seeded players before the draw would be Berdych, Ferrer (basically even), Wawrinka, Raonic. In terms of the danger they present if they get there I would rank them Berdych, Wawrinka, Raonic, Ferrer. Berdych is potentially a quite difficult opponent for Federer and Murray, but a rather easy match up for Djokovic and Nadal, the others would normally be distinct underdogs against any Big 4-player.