I think the distinction between a sampler and a wavetable synth being if the samples repeat after one cycle/period or not is very clear. Other distinctions (like if it is able to record a sample) aren't waterproof and have been blurred as new products have entered the market.
Most probably, that chip was already pretty advanced for its time. Today you can get pretty advanced synths/delays/reverbs on a chip, but they're also generally costly. Example of a lower end FX chip. There might be bucket brigade chips on the cheap, but i don't know. The problem here is for a commercial, easy to source part, the legal dimension of emulating it can potentially get tricky (it's someone's product). For old stuff, you can't reliably source them. So i think there's two options: Emulate and program a suitable DSP pic / mcu / fpga, or make a circuit from more discrete parts (might be too expensive and labourous) and emulate that in turn. Or a combo/compromise.I'm not sure if all this stuff would've made the chip expensive in the late 80s to early 90s though.
Some FX are both cheap to build discrete, and easy to emulate. If you want do give each or some channels a suboctave side channel, all you basically need to use is optionally a comparator and essentially a d-flip flop. Especially if the sub channel is directly derived after tone generation (where the amplitude is known) and then summed/mixed in prior to any volume envelope (and filtering, if that applies). Mixing would require a programmable mixer, of course.
FM/AM synthesis is good in retro gaming (i think) because it offloads the CPU by some (but with 65816, would it matter?). Besides the audio range modulation, you can create vibrato and tremolo just the same way without continously feeding new pitch/volume data to the unit, and tremolo/vibrato would work independent of their famitracker-style instrument envelopes (apart from you wanting to change rate and amplitude of those effects).