Keep in mind, there's a difference between hard and things that aren't good for the map / gameplay.
Using the default playstyle as an example, lets use a pattern like ddKdd. This requires left / right / left+right / right / left. The reason this is unrankable is because it requires you to switch from 1/4 alternating to 1/4 one handed in order to get full points, and at higher bpms / in streams this isn't possible. A more extreme example of this would be DdDdD as it requires 1/4 on one hand for the full 5 notes. As a ddkk player I could play this, but could a kddk player play something like that extended to say 10 notes in a 1/4 stream at 240bpm? Another (main) reason this isn't rankable is the overlapping and hitbox errors that can be caused due to finishers mixed into streams.
As for slider velocities, there's technically no unrankable SV, but the SV should still fit the pacing of the song, and it just so happens with BPM multipliers that 1.4 and 1.6 happen to be the best pacings in most cases. Don't expect to rank a song with hyperspeed SV all the way through as it doesn't fit the songs pacing.
Using the default playstyle as an example, lets use a pattern like ddKdd. This requires left / right / left+right / right / left. The reason this is unrankable is because it requires you to switch from 1/4 alternating to 1/4 one handed in order to get full points, and at higher bpms / in streams this isn't possible. A more extreme example of this would be DdDdD as it requires 1/4 on one hand for the full 5 notes. As a ddkk player I could play this, but could a kddk player play something like that extended to say 10 notes in a 1/4 stream at 240bpm? Another (main) reason this isn't rankable is the overlapping and hitbox errors that can be caused due to finishers mixed into streams.
As for slider velocities, there's technically no unrankable SV, but the SV should still fit the pacing of the song, and it just so happens with BPM multipliers that 1.4 and 1.6 happen to be the best pacings in most cases. Don't expect to rank a song with hyperspeed SV all the way through as it doesn't fit the songs pacing.