Are all sequential circuits based on cross-coupled NAND or NOR gates?
By sequential I assume you mean a (bit) storage cell?

A static storage cell must have two elements:


positive feedback, to maintain a stable state forever
an input that can overrule the feedback (in both directions!)


Cross-coupled NAND or NOR gates have both properties. (The cross-coupling is the positive feedback path.) There are more complex building blocks that can also have these properties (two multipexers for instance), but one could argue that those are themselves composed of NAND and NOR gates.

An other circuit that has this property is an amplifier (call it a non-inverting gate, or two inverters in series) with a weak feedback, and an input that can provide a strong drive to high or low (or be in high impedance, letting the feedback do its work). Whether this counts as a double NAND gate with feedback is IMO a philosophical question.

In the non-static world there are options that store something for a limited time, and then read it back before it fades away, and restore it to its original level. You could see this as a positive feedback loop in the time dimension. DRAMs work this way, but various older techniques too, like storage screens and mercury delay loops.

In the physical world there are some constructs that are bi-stable without an obvious feedback loop. Think of a ferrite core that can be magnetized in one of two directions, or a latching relay that uses a pulling coil to be stable in two positions. The essense here is that there are two lower-energy levels, separated by a high-energy barrier.

Within the digital world I think the answer to your question is YES, but the above examples shows that there are other implementations.