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I like to think of it as the devices being focusing and amplifying systems - the user is capable of doing (some) magic without them, but the magic becomes much more powerful and easier to use when the device is helping.
We know even young Nanoha could do a single Divine Shooter ball when she was training with RH, without RH even out. But when RH is helping her, she can move several of them with much greater dexterity. (Of course, that's the whole point in why she was training - and presumably the effort and exertion she put out when doing it solo meant that she could really rip when she had help!)
Similarly, Nanoha pasted Tea with Tea's Crossfire attack without RH's help. It's not surprising that a (limited to AA) mage with that kind of experience can pull off something that a B-ranked mage needs a device and a cartridge to fire...
(Of course, the question is, how much help can RH give without RH actually being "out"? We don't really know, but we don't see characters pulling off anything really powerful without their devices, so it's hard to tell.)
Yuuno HAD RH, but obviously he couldn't use it well, despite a generally good competence in a wide variety of support magic. We can infer from this one of two things - either devices have varying needs for magic intake, such that a weak mage cannot use a strong device to its full extent... or devices have specific compatibilities, and for some reason, Yuuno and RH just didn't get along. I'm leaning towards the former - even young Nanoha had a lot more raw power than RH, but the manga described RH as a serious drain on Nanoha's magical power until she got used to it. (None of this addresses WHY Yuuno had RH, a device way above the power he could use, on his person at the time. Maybe it was a family heirloom or something? Something he just happened to dig up? Ooh, plot potential.)
We don't have a lot of examples of specific device functionality, though, mostly because virtually all of the magic use we see is related to a one-caster, one-device standard. The addition of the cartridge system to Fate's and Nanoha's devices opened up a bunch of "this plus" type of attacks, and an additional form with additional attacks, but nothing TOO radically different from their former capabilities... more or less "what we used to do, but more of it!" Subaru's upgrade to the Mach Caliber was of a similar style - not really new abilities, at least that she's displayed, but what she had got a lot easier to use to a fuller extent.
Our biggest counter-example is the Durandal, which has a capability not present in Chrono's previous devices; but since we don't really see Chrono in action with it, we have no idea if it can also do the sort of attacks that he's displayed previously.
I'm leaving the Book of the Whatever out of this, because as a Lost Logia, it's clearly an outlier - we can expect it to have all sorts of capabilities that aren't present in other devices in general. How many of those features have survived to be enshrined into Rein Zwei, well, we don't really know yet...
What we cannot clearly explain is what happens when a mage uses someone else's device, or even if it's possible; nobody's lost one yet and they DON'T trade off. Nor do we know how familiar-type magic differs from normal device-assisted magic, though if I can speculate, it's entirely possible that the familiar's construction takes the place of the device - that is to say, that Arf and the Lieze twins can use magic like they had a device because they were made that way. (Zafira's a special case, because he was made in a completely different way, yet it's entirely possible that he's the Velka equivalent.)
And none of this can account for Yuuno's abilities in general, which are pretty weak in attack, but pretty wide otherwise. (Of course, the other characters have decent support magic too - Nanoha can heal and search, Fate can teleport between dimensions on her own power. But we see them in attack way more often, so that's how we think of them.) It's possible that Yuuno's spells rely much more on esoteric knowledge than pure horsepower, though - the fact that he's a scholar of magic means he can make his little AE86 outrun Nanoha's GTR on the Akina downhill, if I can mix an anime metaphor here.
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