I think he's a grittier fighter than Oliviera and carries enough power in his hands to hurt him, I also believe he has better wrestling and is much more proven against high-level grapplers (CDF), has Oliveira beat any good grapplers in his entire career?
Oliveira while is the slicker submission snatcher, I believe Beneil is the more methodical of the two and won't make a lot of positional mistakes, and his improved wrestling should help a lot in maintaining a smothering position.
Oliveira did beat Ferguson though, but I would classify Ferguson as an opportunistic submission artist rather than a high-level grappler mainly because of his poor positional awareness.
the most glaring issue for Dariush is his striking defense and his willingness to walk into fire to get those takedowns and Oliveira has been a sniper lately.
Oliveira has faced tons of high level grapplers from Div I wrestlers, to some of the highest regarded black belts. Actually, he may have faced more grapplers than any other type of style.
Hatsu hioki, VERY high level BJJ, once considered one of the best submission grapplers at FW: subbed by Oliveira in a grappler's wet dream fight.
Nick lentz DIV I wrestler: Look at the grappling exchanges between Lentz and Islam, then compare them to oliveira submitting Lentz.
Kevin Lee: Obviously a good grappler, but lacks in heart and cardio: subbed by Oliveira
Jim miller DIV I wrestler, also bjj black belt: Oliveira finished him on his recent run
Tony ferguson, NCWA wrestler, bjj black belt: oliveira tooled him
Darren elkins, NCAA div II wrestler, Brown belt: subbed by oliveira, and oliveira is the only one ever to do that.
Clay guida, pretty much only wrestles and made a UFC career off of it: subbed by oliveira
Myles Jury, legit bjj black belt: subbed by oliveira