I've used everything.
SSelph is fantastic but has limits, no way to scrape computer systems, horrible snaps for Arcade with all the filters, they are not those you get with other programs and same source. Overall great and fast for console.
UXML scraper is very good but incredibly slow on server. Work only on Win.
Skyscraper, i don't get much the hype, it's good and you can run from Pi terminal, so is slightly fast (not much) and give some very quality snaps (i don't like the cover mix layout), but he missed roms often.
Skraper - Very very good, it's fast but got some problems with some formats, like pbp (very slow), some miss here also, but generally is accurate, and great snaps (those you get around on pre made packs and images, mostly), works only on Win.
Internal scraper - It's pretty horrible, unstable, very low accurate, everyone hate it, but is functional and easily will get the spot, as long the filename is very true to original. Lot of errors - I prefer wrong snap than empty space - i don't recommend folder and overall scraping, very, VERY fast after last update. I would keep it as a last resort, but it has its advantages.