Cut open a chicken thigh that spent all night in a marinade and the center tastes almost exactly like it would have raw, minus whatever seasoning was already in the meat. A brine reaches that center. A marinade almost never does, because the acid and oil doing the work can only travel a few millimeters into the muscle before time runs out.
Why salt gets in and a marinade doesn't
Salt dissolves into ions small enough to diffuse straight through muscle tissue given time — that movement is covered elsewhere on this site, so one line is enough here. The acid, oil and aromatic compounds in a marinade are a different story. They're far larger than a sodium or chloride ion, and diffusion through packed muscle fiber falls off hard once a molecule gets much bigger than that. A marinade never really enters the meat the way brine does. It sits on it and works from outside.
What the acid is actually doing at the surface
Lime juice on raw fish and heat on a sauté pan are doing something close to the same thing: unfolding proteins. An acidic marinade denatures the muscle proteins it touches, and the layer it touches turns opaque and firmer, the same visible change a ceviche goes through without ever seeing a stove. That's the tenderizing effect people credit to marinades. It's real, and it's confined to a thin band at the surface — a few millimeters deep, roughly the same whether the meat sits for twenty minutes or twenty hours, because the acid can't travel any further into dense tissue than that.
Leave it going too long and the same reaction keeps eating into that surface band rather than reaching new territory. The proteins there unfold past the point of holding any structure, and what should have been a light tenderizing turns the outer layer soft, pale and slightly slimy — mushy on a steak, outright disintegrating on shrimp or a fish fillet, which have almost no thickness to spare.
How long is too long
Delicate, thin-cut proteins have the least room for error. Shrimp, scallops and fish fillets can turn from firm to mealy in well under an hour once a real acid is involved, so fifteen to thirty minutes is the useful window, not an overnight soak. A flank steak or chicken thighs can sit for several hours without the same collapse, because there's more meat behind the affected layer and the ratio of ruined surface to usable interior stays small. Marinades built mostly on oil and aromatics, with little or no acid, don't carry this risk at all — nothing about oil denatures protein, so those can rest overnight without the texture going anywhere.
Salt runs on the opposite clock. A properly timed dry or wet brine keeps improving the meat as time passes, right up until it's fully seasoned through, because the thing moving through the meat is small enough to keep going. A marinade stalls almost immediately at the surface and then starts working against itself the longer it sits.
Telling it apart by look and feel
A marinated piece of meat that's still in good shape looks and feels close to raw at the center when you press it, with a thin, slightly firmer rim where the acid has been working. Push a finger into that rim and it should still spring back, not squish. Once it's gone too long, that outer band turns visibly pale or opaque all the way around, feels soft or slick rather than firm, and can carry a faint sour edge on the nose beyond the marinade's own smell — a sign the acid has been sitting on that surface longer than it should have.
Reusing the liquid
The marinade a raw cut sat in has picked up whatever bacteria was on that meat's surface, and it needs the same heat the meat is about to get before it's safe again. Boil it hard for a few minutes and it can go back on as a sauce or a basting liquid partway through cooking. Pour it over food straight from the bowl and it's carrying the same risk the raw meat did — the acid in it does nothing to change that.