Advantages

Frequency Resolution
Long word length phase accumulator can provide frequency resolution to sub micro-hertz levels.
(In a variable-clock arbitrary system, frequency resolution is defined by the clock generator which could itself use DDS).

Digital Modulation
Modulation words can be added to the phase accumulator to provide digital modulation.
(Direct digital modulation is not possible in a variable-clock arbitrary system).

Automatic down-sampling
Waveforms are automatically truncated by sampling to allow repetition rates above
Fclk / Waveform length.
(In a variable-clock arbitrary system, waveform frequency always equals Fclk / Waveform length. When Fclk is at maximum, shorter waveforms must be used to obtain higher frequency).

Disadvantages

Waveform Jitter
Only frequencies equal to the clock frequency divided by the waveform length (and sub multiples) are un-sampled. All other frequencies are up-sampled or down-sampled resulting in missing samples and/or jitter.
(A variable-clock arbitrary system is inherently free of sampling jitter).

No Sequencing Capability
Only a single waveform can be generated at a time. Memory segmentation and waveform sequencing is not possible.
(Most variable-clock arbitrary systems incorporate extensive facilities for waveform segmentation and sequencing).

No External ARB clock
DDS generators can not use a variable external clock to clock out the waveform.
(DDS generators use a fixed frequency clock. Although this can be locked to an external signal, an external signal can not be used to clock out the arbitrary waveform. Some variable-clock arbitrary systems allow use of a fully variable external ARB clock).

Many real-world arbitrary waveform requirements can not be fully met using a DDS generator.