I suspect that many dealers and gunsmiths don't like the 710 because it doesn't provide them with the potential "after the sale" income. There's probably also a lot lower profit margin on a 710 and other entry-level models. Who's going to sink a lot of money - such as a high dollar scope or trigger work - into an entry-level rifle?

I've heard a few claims about internal plastic parts on the 710, but never seen or read anything before about plastic firing pins. My 710 has the nylon insert in the receiver, but I believe the newer models are all steel and do not have the insert. The 710's magazine is metal, not plastic. The base plate on the mag is plastic as is the follower, but the housing/box itself is metal.

As for the trigger guard being made of "plastic", let me say one word: "Glock". The 710 is by no means in the same league as Glock, but these "plastic" guns were derided 20-something years ago as "tupperware" and other derogatory terms because some thought it sacrilege for a firearm to be made of "plastic". I'm not going to take a hammer to my 710's trigger guard to see how much pounding it can actually take, but I imagine it's not something that will easily break.

I would bet that whatever force might end up breaking a synthetic trigger guard would also possibly bend a metal one into the trigger and possibly damage it, so arguing over a "plastic" trigger guard seems to me to be nothing more than nit-picking.
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Forgot to mention that I've had a 710 for 3 years now and found it to be quite a competent rifle. It shoots nice groups, has worked flawlessly and taken a couple of deer. I got mine in .30-06 because it would handle anything I might ever get the chance to hunt and my duaghter can use the Remington Managed Recoil loads when we hunt together.