Firbolg

[Graeme Dice]

While a wraithlord costs the same as 4 summonings of tartarian titans, it will also easily defeat all four in combat if they aren't commanders.

[Manuk]

But nature gems are always in the need. Gift of reason costs 20 gems, ivy kings cost a bunch of gems and if you are clamming costs nature too. You also may need regenerating rings or just vine ogres.
Death gems burnout quickly too, too many uses to them.
On the other hand, Air gems don't have too many uses.

[PvK]

A Wraith Lord may be better than a Tartarian non-commander unit for some things (or not - undead Manticore for 10 death gems, anyone?), but a Wraith Lord costs much more than a Tartarian.

(Sleepers are good thugs as well as good leaders.)

[PvK]

Seems like the 4 Tartarian units will tend to be stronger, to me, except for the immortality part.

[Shovah32]

Yes. But if the wraithlord used some anti-undead death magic their extra punch in combat might not help.