ReadCube Papers has been around for a while now. I will focus on one aspect: matching.
The latter worked pretty well in Papers 3, although some implementations, such as with Zotero are still superior, in my opinion. Finding and retrieving meta data is important since manual entry is a time-consuming task. Now, often, the data needs fine-tuning but a basic look up is highly desirable. ReadCube Papers fails miserably at this task. Unless there is a readily detectable DOI, RCP finds nothing. If you have mostly recent science and medical papers, that might not bother you that much, but for people who are in humanities and also add books, it's a desaster. They recommend putting author and short title. That has never worked for me, not ever, not a single time; I've got thousands of papers. RCP doesn't give you look-up options, i.e. specify libraries etc. Zero. Even entering a DOI manually or an ISBN returns no results. None. Compare that with Zotero that gives me a 90%+ return on ISBNs.
This is just one aspect, but a crucial one. This is unacceptable. I'm currently giving ReadCube Papers a second try and have even bought a plan. It syncs well with iOS, that's really the only positive aspect. However, if the matching desaster isn't bettered, I'll be out again.
A final tidbit, whenever I quit RCP (normally), MacOS reports it 'crashed'. What does that tell you? They can't even get the shut down right. This is banana-software. Sold unripe to the customer, left there to ripen over time. It's been such a long time now, that the banana is becoming foul and putrid.