L2TP is affected by packet header modification and cannot cope with firewalls and network devices that perform NAT.
“PPTP can run only on top of IP networks.” is correct as PPTP encapsulates datagrams into an IP packet, allowing PPTP to route many network protocols across an IP network.
“PPTP is an encryption protocol and L2TP is not.” is correct. When using PPTP, the PPP payload is encrypted with Microsoft Point-to-Point Encryption (MPPE) using MSCHAP or EAP-TLS.
“L2TP supports AAA servers” is correct as L2TP supports TACACS+ and RADIUS.
NOTE: L2TP does work over NAT. It is possible to use a tunneled mode that wraps every packet into a UDP packet. Port 4500 is used for this purpose. However this is not true of PPTP and it is not true as well that it works well with all firewalls and NAT devices.
References: All in One Third Edition page 545 Official Guide to the CISSP Exam page 124-126