Sounds like disciplinary issues waiting to happen. You're forcing millions of people who don't want to be there, may have better options, or both to do military service. It's when and how many, not if, people aren't going to be willing to play ball. Not to mention most 18 year olds are not qualified to be in the military.
Military Times article
At least some of these people will have talents that can be put to better use elsewhere, you're delaying their ability to do so. It's also going to cost a bunch of money, those people have physical needs, that will cost tax dollars to fulfill, and those people are going to want paychecks. They're not going to be happy if they could get higher pay elsewhere and are barred from doing so.
Many disciplinary issues stem from supervisors letting the small things go, the stuff that doesn't warrant action by the commanding officer. People with disciplinary issues or just bad attitudes will have them no matter where they are or what they are doing.
If we want a qualified pool of individuals, mandatory registration and calling up a portion of them as needed will do. Affects to careers that are put on hold can be mitigated by law just as there are laws in place now to protect service members and their family financially. Remember, for the vast majority of the human population, careers in the hard sciences are not going to be disrupted. Most people are just filling cubicles.
Plenty of countries have mandatory conscription Switzerland, Norway, Greece and Finland which I'm told is the happiest place on earth. Finland is six months to a year. That's a high turnover rate and they seem to find the money for it.
Peoples perception of the military must largely come from movies, that there needs to be some warrior attitude to serve. Or maybe it's piss poor marketing on the part of the military. It floors me that people willingly take on a mountain of student loan debt that will take them on average 20 years to pay off when for three years of active duty between high school and college the government would pay for it all for four years and provide a monthly tax free allowance along the way. Here that allowance would be about $1400, in Southern California, about $4000. For most, this is way better than mom and dad can do.
Disciplinary issues aren't going to go away by being more strict; it might make it worse. The issue I'm talking about is resistance from the people who don't want to be there. Some portion of them will do just enough to keep their immediate supervisor off their ass, usually a Staff Sergeant or Tech Sergeant (USAF ranks). Not their best work, just enough to stay out of trouble for a 4 year enlistment, plus an IRR component. Others will actively do stupid shit that might require the 1st Sergeant or the CO to be involved.
No amount of laws will remove the "I don't want to be here" problem that will occur with any type of conscription.
I know what it's like to be in the USAF in the 1990s. I know that most people don't see combat, I had an office job. I'm also aware of the GI bill from that era as well.
Mandatory military service may not have anything to do with those countries' citizens' happiness. That whole correlation/causation thing.
As for recruiting, military recruiters are allowed in all high schools and colleges in the USA by federal law. LIke the article I liked earlier, most teens and young adults aren't qualified for the military. Many that are, aren't interested. Ads on TV, national anthem at most, if not all, sports events, flyovers at some sports events, pledge every day in many schools. Some places even give discounts to active duty, and/or veterans. It's not for lack of recruitment effort.
Money, about 4.2 million kids were born in 2007. They turn(ed) 18 this year. That's more than all branches of the military combined, including guard and reserves. If some are being conscripted, and others are not, resentment will build among the conscripted.
https://www.cdc.gov/nchs/data/nvsr/nvsr58/nvsr58_24.pdf