Yunohost is probably more secure than you figuring everything out yourself. More people have a vested interest in keeping it secure. They have a minimal page on security but they have fail2ban, unattended upgrades,and a secure SSH configuration. If something is discovered, you might be vulnerable but at least there will be knowledgeable people fixing it.

Security is always difficult and nothing is 100% secure. The three letter agencies around the world have been hacked and they are in the business of hacking others. Hackers themselves get hacked on the regular. Using yunohost as a noon probably reduces the chance of you getting hacked.