Keycap “universality” is mostly not determined by whether a switch is optical, magnetic (Hall effect), or electrocapacitive (EC). It’s determined by the mount/stem standard and a few geometry details (height, stabilizers, LED orientation, and low-profile spacing).

1) Optical switches (Light / Infrared)

Most optical mechanical switches use an MX-style “+” stem, so standard MX keycaps usually fit.
But watch for two common exceptions:

• Low-profile optical (slimmer switches/boards) often uses non-standard low-profile keycaps.

• Some brands use proprietary stems or unusual key spacing—then normal sets won’t fully cover the layout.

Keycap advice (optical):

• If the switch has an MX “+” stem, choose any MX-compatible keycap set (Cherry/OEM/SA, etc.).

• Pay attention to north-facing LEDs (possible interference with Cherry-profile on some boards) and stabilizer type.

2) Magnetic / Hall effect switches

Hall effect is also a sensing method; many Hall effect boards still use a mechanical stem, often MX-style “+”, so MX keycaps often work.

Where people get burned:

• Some Hall effect keyboards use proprietary stems or low-profile Hall switches → then keycaps are not standard.

• Some rapid-trigger boards have tight tolerances; heavy/tall keycaps can change feel (not “incompatible,” just “worse experience”).

Keycap advice (Hall effect):

• Prefer MX-compatible keycaps if the stem is MX.

• For best performance and consistency, consider medium weight, consistent wall thickness keycaps (extremely heavy/tall caps can feel sluggish or amplify wobble).

• Check stabilizers and layout coverage carefully (spacebar size, right shift size, etc.).

3) Electrocapacitive (EC) — most commonly “Topre-style”

This is the category that is often NOT universal:

• Many EC boards (especially Topre-style) use a Topre mount, not MX.

• Standard MX keycaps will not fit without an adapter, and adapters can introduce wobble, height changes, and uneven sound.

Keycap advice (EC/Topre):

• Best experience: buy Topre-compatible keycaps made for your exact layout.

• If you must use MX keycaps: use Topre-to-MX sliders/adapters, but expect trade-offs (fit, sound, stability, height).

Professional checklist: how to choose correctly (fast)

1. Look at the stem shape

   • MX “+” → most MX keycaps fit

   • Topre mount → Topre keycaps (or adapters with compromises)

   • Low-profile / proprietary → requires matching low-profile/proprietary keycaps

2. Confirm stabilizers

   • Cherry-style vs Costar vs Topre stabilizers affect large keys (space/enter/shift).

3. Check LED orientation & profile

   • Some boards with north-facing LEDs can have interference with Cherry-profile on certain switches.

4. Match your layout coverage

   • 65/75/96/1800, 1.75u right Shift, 7u spacebar, ISO enter, etc.

Bottom line

• Optical & Hall effect: usually MX keycap compatible, if the stem is MX and the board isn’t low-profile/proprietary.

• Electrocapacitive (Topre-style): often not MX compatible; use Topre keycaps for the best feel and stability.