Several countries have 2-3 years compulsory military service after high school. This is just a volunteer version of that. If you want to do it, then do it, no one is forcing you to join it and become unemployed after a few years. I assure you no one plannning to become a software dev or engineer or doctor or accountant or finance professional will chose it and ruin his career. The only people who will go for it is those who either really want to join the army, or those who didn't want to go to college to further study anyway, or maybe those who wouldn't even get into any mediocre college with the grades they have. So this is an opportunity for those who don't wan't to further study and have no plans to at least do something with their life, join it for a few years, earn some money, build fitness and then if you are really good, you will be retained after few years.
The biggest disadvantage of this scheme is that those who really wanted to join the army now have no guarantee of employment. Only 1/4th of the people who join the scheme will be made permanent, so imagine those who have been trying for last 2-3 years to get in and have planned their enitre lives knowing that they will get in the army, they have spent last few years trying to meet the physical/educational parameters, and if they knew about this earlier, maybe they would have opted for another path... so obviously they are devastated and really really pissed and rightfully so.
Another disadvantage is that this scheme risks bringing in low morale people at the cost of people who really wanted to join the army. Imagine someone who doesnt really care about the army, but he performs better in the 4 years than the guy who really wanted to join for a lifetime, so after 4 years, the guy who really wanted to join will lose his spot to the other guy, which is really not good.
Advantage of the scheme is more money for acquiring heavy weapons and Research and development, since currently most money goes to pensions and salaries.