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Only problem with a loop seal is if someone jacks the pressure up. Other than that, more reliable than a trap.

You're not missing something. The combination of loop seal and crossover trap is much cheaper and simpler — and considerably more reliable — and offers higher capacity than an F&T.

However.

It this condensate path is a loop seal to the dry return, as it sometimes is, it is limited to relatively low pressures. Even if the loop seal is a generous 60 inches, any steam main pressure over 2 psig will allow steam to blow through the loop seal and pressurize the dry return. This is not good.

Now if the loop seal isn't really a loop seal, but consists of two drips (one from the main, one from the dry return) going to the wet return, this limitation doesn't apply — but… now if the boiler pressure acting on the wet return will raise the condensate in the wet return up to the dry return if the pressure is too great — using that 60 inches, 2 psig again.. This isn't good, either.

So. Bottom line: if you boiler pressure is much over a couple of pounds, you are going to need an F&T. Otherwise, you're better off with a crossover trap and a loop seal or double drips.

Note that an F&T makes a truly miserable main vent — so you'll need a main vent at the end of the steam main.