The Russo's career pre-MCU was far more impressive than Todd Phillips'. Yes, the Russo's didn't have the most impressive movie track record, but their tv stuff was very well received. I mean, they were one of the parties behind the start of what is considered by many to be one of the best comedy shows of all time (Arrested Development). And of course they were one of the main creative forces behind Community, a show that is also loved by many and a critical darling.
Todd Phillips' most notable career achievement is co-writing Borat and, uhm, giving the world The Hangover, I guess.
There's also the fact that this seems to be DC doing a Logan-esque film (where the director basically has full say over what kind of movie he wants to make), which is vastly different than the MCU Russo films, which have been mostly devoid of a personal touch. It's very obviously trying to evoke classic Scorsese films, which is a type of film Todd Phillips has never, ever made before, so of course people are a bit wary considering (almost) all Todd Phillips has done so far are middle-of-the-road comedies and not strong character drama.
Now I'm also looking forward to Joker, the trailer looks interesting and the concept sounds good, but comparing Phillips with the Russo's is weird. This movie seems to be something totally different than what the MCU is going for.