5 of the RME won't cut it.
It's actually 7 if taking the Bass/Treble ones into account which can be changed just like the others (minus some limits regarding frequency range etc). Still not 10, but depending on application it can be enough.
There is no difference in sound quality EQing with Roon vs. RME ADI-2
IIRC the Adi manual mentions which types of filters are used exactly, and these are known fairly standard mathematical equations turned into software, so if Roon uses the same filters then given the same parameter sets bot should produce the exact same output. Even if not, it's hard to talk about this as difference in 'quality': a different type of filter yields different sound but it's usually not possible to qualify that as better or worse - just different.
tldr; as far as sound goes I'd say it doesn't matter, so what's left is mostly preference/convenience of application. E.g. without EQ my headphone isn't all that so it would be weird for me when depending on the software I'm using, the sound is different. So even if I'd not be using EQ on the Adi I'd still use Equalizer APO not Roon's EQ so that when I occasionally play through youtube/soundcloud/whatever, EQ is still applied. Also changing EQ and naming presets etc on the Rme isn't exactly the most user-friendly experience so when creating new EQ I almost always do it on the PC first and then when satisfied make a new preset on the Adi.