The Raven Queen Warlock in my group got a Sentinel Raven, as per the Lvl1 Class Feature. This raven is now attuned to a magic ring and wields a magic wand. The wand is not that big of a problem, but the ring allows the raven to cast Earthen Grasp and Wall of Stone, amongst others. (Both are homebrew items.) This has led so far to some pretty awesome moments, but also to problems. That's because this Sentinel Raven cannot be targeted by harmful effects or spells and can't take damage while perched on the warlock's shoulder.
While perched on your shoulder, the raven can’t be targeted by any
attack or other harmful effect; only you can cast spells on it; it
can’t take damage; and it is incapacitated.
So it just flies off the shoulder, does some magic, then lands back on the shoulder in the same turn. This makes it (virtually) impossible for me as the GM to break concentration on spells like Earthen Grasp. That's really annoying sometimes and it makes balancing encounters rather difficult.
Previous answers seem to agree that familiars can indeed attune to and use magic items (Can a familiar attune and wear or use a magic item?). Plus, the magic items in question can be used by a spirit that assumes the form of a raven, as they don't have any constraints that would restrict their usage and the spells cast from them do not require any components.
Is this an intended feature of the Sentinel Raven? Am I overlooking something? How can I solve this problem in the game and/or at the table?
The warlock player really likes the raven and is a huge fan of the versatile magic it can do now with the magic items he gave it. I don't want to punish anyone, just find out if this is legit and what to do if it is not.
Yes, you are missing something; an incapacitated creature cannot maintain concentration:
Once the Raven lands on the Warlock's shoulder, Earthen Grasp ends as it becomes incapacitated.
The conversation with the Warlock player
Obligatory warning about UA. Unearthed Arcana is playtest material, just as the UA content requires a DM's nod, it also requires the DM to balance the content in their game when they see something too powerful, so you are well within your power to disallow any weird interactions with UA. You might even want to send feedback to WotC about it.