Disadvantage of having a short sprue is:
SPRUE LENGTH
The length of the sprue former depends on the length of the casting ring. If the sprue is too short, the wax pattern may be so far removed from the end of the casting ring that gases cannot be adequately vented to permit the molten alloy to fill the ring completely. When these gases are not completely eliminated, porosity may result. 
Therefore, the sprue length should be adjusted so that the top of the wax pattern is within 6 mm of the open end of the ring for gypsum-bonded investments. With the higher-strength phosphate-bonded investments, it may be possible to position the wax pattern within 3 to 4 mm of the top of the investment. For reproducibility of casting accuracy, the pattern should be placed as close as possible to the center of the ring. However, the position of the wax pattern for vacuum pressure casting may be different.
Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 214,215
Rapid solidification of metal
No place for reservoirs
Incomplete evacuation of gases
Difficulty in removing casting from investment