Muscle failure isn't required to build muscle.
When you lift something heavy, even once, it forms small tears in the muscle. Your body repairs those tears and gets stronger in the process (think about it like breaking a bone where it forms more bone over the break site). This is also the reason I tell people that what you do in the gym is only about 10-20% of what matters. Diet and rest make up the bulk of your progress. If you don't provide your body the correct nutrition to repair the muscles, it won't happen, and if you don't get enough sleep, you're also fucking it up. There's a reason people (non-steroid users) don't go and do two a days 7 days a week. It's just not necessary.
The free weight versus machines thing has been covered pretty well already. The stabilizing systems around your muscles (tendons, ligaments, smaller muscles) get stressed more when you use free weights. For the compound lifts, this also includes things like your core (abdominals, rhomboids, lats, spinae erector) which basically a machine can't help you with. Sure, you can work those muscle individually on
anothermachine, but that's incredibly ineffecient. Machines definitely have a place, and the longer I lift the more I use them, to be honest. But free weights (at the very least for compounds) are incredibly important for developing the "everything else" so that you're not imbalanced as much.
Finally, your concern about injuries is ridiculous. As long as you ensure you have good (doesn't have to be perfect/great) form and you're not ignoring signs from your body that you're doing something wrong, injuries aren't something you should be concerned with. It's when people start piling on more weight than they should (ego lifting) or ignore something like a shoulder impingement for months that you run into problems.