Invisible players on the other team has been a problem since day 1. Alt+tabbing in full screen mode, internet problems, or hardware problems can cause this glitch. Any time you feel stutter, just press a bind that records a demo and stops it. (It is especially useful if you have this problem a lot, but everyone should have this bind handy just in case) Valve had said they fixed this issue a while ago, and they did, but I believe it came back with a vengeance.
The invisible players with cosmetics I've found has been a problem with spectating. If you stay in third person you will never see an invisible model, but if you stick to pov of one person they will constantly be invisible. (which is why you will never see an enemy with floating cosmetics in 6's because you can't spectate the other team) Not sure how to fix this client side without dying and respectating them and switching povs. If that person dies, they will become visible again as long as you aren't still spectating them.