Frame flex is a non-issue firing off-hand. The bullet is out of the barrel before anything flexes. Now, if you pin the grip rigid in a gun vise, it's a little different, but it's a non-issue fired off hand. Off hand, the polymer absorbs some of the shock, but there's no flex reducing mechanical precision. Metal receivers have no advantage in this regard.
Metal pistols feel smoother because they're heavier, and mass is the primary mitigating factor against recoil.
More convertible in terms of grip variety? There's a sleeve of every contour to match whatever you might do to a metal-framed pistol, there are easy stippling options, and many polymer pistols come with multiple grip inserts to change grip size and profile. On the whole, this is clearly an advantage for polymers over metal frames.
The notion of them handling more rounds over years of use seems like one of those assumptions that is so widely held that it's treated as fact. The slide and barrel are what get abused in terms of long-time durability. And the barrel and slide are steel on both types of pistols. I wouldn't assume the material of either frame lends itself to long-time durability, unless, of course, I factored in corrosion, in which case -- advantage polymer.
The extra mass of an all-metal pistol is an advantage in a number of applications, as illustrated by Joe L, but it's a disadvantage in others. Durability is a myth at worst and an assumption at best. How long a pistol will function reliably and how long a part will function without breaking is matter of design, not whether or not the receiver is polymer or metal. The 1911 is the god emperor of metal pistols, and it's not know for impeccable reliability. It's known for needing tune-ups.
I'm not downing metal. It's just a non-factor in most of the reasons we give when explaining why we prefer one over another. It's all bias and arbitrary preference and, honestly, FAR too often, it's ungrounded romantic sentiment.
