I totally agree with Emma (@UniversalPA). I tried both in 2017.
Upwork told me that I was not able to apply as there were already too many people offering the same skills I was and they did not want to saturate the market (I had not followed their advice to pick a niche … still my bugbear!) and PPH accepted me right away but were awful … definitely the cheaper end of the market and not real quality work being done.
Another problem I had, was that these sites required providing proof of passed work most of which I am not allowed to share so was really rather stuck when writing bids.
That doesn’t mean it can’t work for a little extra income, but in my experience I spent way too many hours looking and applying for jobs and got very little in return (though I am thinking of using it to get very cheap vectorised logos created only to save me the trouble of working out how to download a font in Inkscape and recreating it myself).
Sorry to be negative, but I think that the time spent applying for jobs would be better spent networking or, dare I say it, posting content on social media (something I still need to think about).
All the best,