Any new grading company has a tough road ahead, how do they prove their grading unless they get the full range of cards to grade? It's fine to grade a load of commons, but that says nothing about consistency. I often feel that cards of star players or the more "iconic" cards get better grades than similar commons. So having some really nice cards graded would be important. If I was starting one, I'd offer to do some high profile cards for free.
Sure, what holder a card is in makes a difference depending on the reputation of the company. They've all made mistakes. Some legit companies have made far more mistakes than others and that's why they aren't as trusted. Just avoid the really sketchy ones. The ones like the guy "grading" magazine cutouts and pop riveting them into screwdowns aren't legit at all.
But to say "oh if it's PSA I know I can trust the card isn't altered" is only a bit more true than saying that of any of the second tier companies.
Learn enough to trust the one hobby person you should put the most faith in -
YOU. The holder doesn't matter, the lack of a holder doesn't matter.
Will you make mistakes? YES. I've been collecting since .....well, not as long as some, but plenty long enough. And I still make mistakes. (Massively overpaid for a stamp this week, but by massively overpaid I mean I paid $5 for a $1 item. ) I've also done fairly well buying ungraded cards even on ebay. It's kinda fun to buy an ungraded common t206 and have it grade as EX
I make a lot fewer mistakes now than I did 20-30 years ago. And fewer still than I made back in the 70's.
Steve B