First of all, the last Metroid game was Other M. So that's probably reason enough to not necessarily trust that the next Metroid game will be good.
That said, I'd argue that you should never 'trust' the person whose job it is to sell you something, that that something is good. It's in their best interests to tell you it's good, regardless of quality.
Although, with that said, I think you're misinterpreting the complaints and/or the complainers are doing the typical internet thing, where, if they don't like something, they feel the need to paint the entire thing as an absolute trainwreck, rather than see it as a thing with good qualities, and bad qualities, that they happen to dislike for whatever reason (see: every thread ever with the title 'why is 'x' so popular'). Basically, people aren't saying Federation Force is bad because they think it's a bad game, they're saying it's bad because it's bandying around the Metroid name, whilst not really being a Metroid game. Which is fine sometimes (Dynasty Warriors gameplay, with Zelda characters? Sure, sounds cool), but when it's been 5 years since the last entry in the series, and that last entry was controversial (to say the least), fans can get a bit antsy, and a wildy different spin off sets them all a-flutter.
Basically, I think you should just ignore the whining on the internet, wait 'til the game comes out, read reviews, watch some gameplay, listen to the general hum of the internet, then decide if it's worth a purchase. 'Trust' should never enter the equation. Even entirely sincere, well meaning, talented game designers can screw up sometimes (see: Metroid: Other M, or to stop banging on about that, every numpty that showed up to see Star Wars I: The Phantom Menace on opening night, boy did the massive amounts of fandom trust blow up in their faces)