[In case you feel I’m being hard on VCS-AMS, the same failings apply to offerings from other major EDA vendors]
Digital-on-top AMS simulators don’t work
For a mixed signal simulator to work properly all the wiring needs to be analog so that all analog things attached to a wire can be computed together. Digital-on-top simulators tend to end up with islands of analog that are not properly connected.
The second problem with having digital in charge is that the timing resolution tends to get quantized on the A/D boundaries more coarsely than if analog is in charge and that will give you timing errors.
Bottom line: AMS simulators need to be analog-on-top because digital is just a degenerate case of analog – there is a spectrum of model accuracy and the 1/0s of Verilog is on one extreme.
A note on speed
Analog simulators use PWL signalling between models, which is in itself a discrete form, they also do potential and flow (V & I) on wires. Since that is a discrete form, a digital simulator could handle it directly, at least for V or I, without significant impact.
What slows down analog simulators is the interaction between models that needs to be solved for with matrix math. Analog models that don’t interact that way (signal flow) can perform at similar speeds to digital 1/0 models.
A note on power
Calculating power in an analog simulator is easy because it has both V & I in its models and wiring, digital simulators (like VCS) only model the V or I (unidirectionally) and can’t work it out for you.
Using built-in A/D conversions
Rather than support connect-modules properly, Synopsys would like you to use their non-portable proprietary built-in conversion elements – you get 4 of them vs user-space programmability. The additional problem that causes is that the threshold/drive characteristics are defined in one configuration file and not within the design, so trying to characterize thousands of different boundaries within an IC is intractable, and accuracy is lost – tools can handle the complexity of Verilog-AMS connect-module approach automatically (it was designed for that). Sharing AMS IP is also somewhat harder if someone in the flow wants to be non-standard.
The driver access functions which help with D2A accuracy are also missing in VCS-AMS.