Since the forward speed is determined by required lift, steeper approach path means higher vertical speed.
Considering approach speed of 130 knots:
- at 3.0°, which is 5.2% or 1:19.1, the vertical speed is 690 ft/min,
- at 3.2°, which is 5.6% or 1:17.3, the vertical speed is 736 ft/min,
- at 3.5°, which is 6.1% or 1:16.3, the vertical speed is 805 ft/min and
- at 5.5°, which is 9.6% or 1:10.4, the vertical speed is 1268 ft/min.
Since normally the criteria for stabilized approach is that vertical speed does not exceed 1000 ft/min, the 5.5° requires different procedure with different criteria.
Vertical speed equals power ($P = mgv_v$ where $m$ is mass, $g$ is gravitational acceleration and $v_v$ is vertical speed) and to maintain speed, the engines have to develop less power by this amount, of drag has to be increased.
This is usually not a problem for propeller aircraft, especially those with variable pitch propellers (which turboprop requires and larger piston-engined aircraft have as well). With maximum RPM selected the propeller produces quite a lot of drag at idle, but since everything is already spinning fast, adding power has immediate effect if needed to go around.
It may, however, be problem for jet aircraft. Jet engines need to spool up to increase thrust and from very low power settings that can take several seconds.
The normal glide-slope is well below the gliding angle with no power (with flaps and gear), so the engines are still running at considerable power. In the slightly steeper approaches the power is less, but the difference is not as big, so it usually isn't a problem.
But the 5.5° steep approach is getting close to the unpowered glide angle even with full flaps and gear and is steeper than unpowered glide angle with lower flaps settings. This means the engines will react more slowly and pilots have to consider it. Also in some aircraft an engine-out approach can't be flown at full flaps, because the remaining engine(s) would not have enough power for go-around at that drag, which means such aircraft can't fly steep approaches with failed engine. And some aircraft may not be permitted to do these steep approaches at all, because even at full flaps the engines would run too slow and take too low to spool up.