Is this the case, should I skip Frontiers?
Frontiers is a pay to publish venue and is often considered shady and predatory, MDPI is usually considered to be below Frontiers. In many university or even countries, Frontiers publications don’t count toward tenure or toward the assessment of researchers and PhD positions aren’t allowed to publish there. A researcher would try to get his articles published somewhere better. You wouldn’t consider it an big accomplishment publishing in Frontiers and researchers are proud people. There’s also so many Frontiers sub-journals that the quality varies and you’ll find some that are very decent, whilst a physicist publishing in “Frontiers in physics” would be looked upon as “your paper got rejected by all reputable journals but you still desperately need it to be published so you paid.”
However, it’s also extremely field dependent and in some fields it’s not a taboo at all and the various journals seem good with reputable authors publishing there, of course without being top journals.
That being said the quality of an article speaks for itself and you can still find good publications in bad journals (and bad papers in good journals). It's the quality of the work that matters.
Personally, what I find a worse practice is if people don’t submit to the Arvix. I find that ridiculous.
