Hi.
Do you have any photos of what you mean by "open action"?
There's a "metal vs wood" debate and "flat vs stepped".
Traditional wooden action has problems related to the dimensional stability of the wooden keys & comb + excessive wear on the key axle holes.
There's ways to mitigate this, e.g. by making keys from laminated wood, inserting brass or ptfe bushings for the axle to go through, etc, but I'm not aware of any vintage or mass-produced models that have these features.
Metal action tends to stay where it is, but it's rare to see metal action with compression springs under the keys...Hohner did it on their top models in 40s and 50s. I might be crazy, but I think that compression springs feel better than expansion or rotational springs, but we are splitting hairs here. Or maybe compression keys are just easier to set up precisely for the right tension.
Flat vs stepped depends on the playing style. For CBA or, say, Russian "Hromka" harmonica, where thumb is used quite a lot, stepped is key. For a 2-row bisonoric melodeon it makes very little difference, as you don't tend to use the thumb that much. No idea how it applies to single-rows.