Wraithlords aren't that bad, even though they cannot advance through terrain in the same way the Guardians can. If you're using the Guardians in more of a support role, then the Wraithlords add a little bit of solid fire support, and an extra dimension in terms of durability.
The downside is that Guardians are horribly brittle when they come under fire, and even if you add Wraithlords, and possibly some heavy or support weapons, they simply don't have the range, or the quantity of shots to last a war of attrition, as I've found to my cost, so you need to ensure that you've got other units to work with them, and not leave them isolated at the back if you're going to use them on foot, and with most Eldar infantry being assault oriented, that means using tanks. This is where Falcons may come in handy, but they aren't cheap, and you may wish to use most of them as Wave Serpents if you are thinking of investing in Aspect Warriors in the future, or of transporting some of those Guardians.