Pros: Tatanggalin mo si Google apps. Ang lakas kumain nang memory and lahat para lang nakawin ang data mo. Replace those app you use with non Google equivalent apps (mxplayer for music,yt vanced for yt,Firefox for Gmail/FB,etc) and bonus Hindi na xa nag background process,ginhawa na memory mo.
Pede I over clock phone mo. Not recommended,pero Kung alarm mo ginhawa mo,pede mo patakbuhin na parang 6gb ung 4gn phone mo(aunog mga lang phone mo,lel)
Pede Ka maglagay nang häçking tools sa phone mo, like WiFi sniffer, and others. Do it at your own,nga lang.
You can install custom ROMs that runs lighter and faster than default os that comes with the phone.
CONS
Kung Hindi Ka marunong,maabibrick mo ung phone mo. Congrats may pambato Ka pag galit Ka.
Tinatanggal mo warranty nang phone mo,sonpag nasira Yan,tatawanan Ka lang pinagbilhan mo.
Pag tinanggal mo ang os nang phone mo,may change na bigla lang mag hang ung phone mo,Kasi sometimes Hindi pa sigurado ung ROM na nilagay mo.
Ang hassle maintenance Nyan pag Hindi Ka marunong mag manual update. Di tulad nang regular na OTA okay na.
I think, that's it? Please correct me if I'm wrong...