Sammy makes impressive looking saves, but it's usually because he's out of position and relies on his athleticism to occasionally bail him out. His positioning is generally bad. His angles are frustratingly off at times. Yes, he can get hot, but his hot streaks don't last long (hence the back-and-forth with Vanacek, who is less talented but plays a more reliable/consistent game). His rebound control is poor and he also has issues tracking the puck. He's also not particularly good at playing the puck; he's had multiple boneheaded mistakes behind the net that have resulted in trouble, if not a goal against. (These are usually the result of poor communication, which is another issue). Normally, you can count on him for one bad to really-bad goal per game.
But, former first round pick, raw talent., etc. etc. Also the Caps goalie coach is terrible, so maybe a better coach can get more out of him. But at this point you're not still counting on potential. He's a reclamation project.