1.What is other Layer7 DDos attacks, e.g FTP, DNS ?
Application layer attacks or layer 7 (L7) DDoS attacks refer to a type
of malicious behavior designed to target the “top” layer in the OSI
model where common internet requests such as HTTP GET and HTTP POST
occur. These layer 7 attacks, in contrast to network layer attacks
such as DNS Amplification, are particularly effective due to their
consumption of server resources in addition to network resources.
2.If I use WAF instead of Layer7 ddos protection, what will be risks in my system?
WAF can mitigate ddos attack in under certain level. Application layer ddos attacks are very difficult to avoid. Specially bot net attacks are not spoofed and request seems to be fully legitimate.
3.Using WAF for ddos protection is a prefered way?
You have to use adaptive technology like a captcha code. Because botnet attacks are very difficult to identified from legitimate traffic therefore captcha will be best solution.
Web application Firewall also option. Please Read link.
(Details copied from cloudflare)