LIttle backtory first;
Our gaming group has been fairly consistent for a few years now; we played Rise , and are now Halfway through Shattered Star; The players are fairly good at optimizing and have a good rule knowledge. We're only 3 and we have no big issue with the difficulty even in a dungeon focused AP like Shattered Star
Now we are planning our character for Return, and the 3 players want to play an Arcane Caster of some sorts : Sorceror, UC Summoner and Witch.
DM who played return, how would you imagine the major roadblocks be for such a group?
Well, since Witch and Sorcerer have d6 HD, surviving the lower levels may be challenging. The first adventure has some undead, which means they may want to avoid mind-affecting options to start - I have an Enchanter in my current PbP of Return, and she hasn't been very effective while exploring Roderic's Wreck. Something for the Witch to keep in mind when choosing Hexes.
Otherwise, there are ways for one of them to fill the role of trap person/door unlocker, and cover healing. The biggest thing I would be concerned about is encounters against multiple melee enemies, as none of the classes is a strong melee class, outside of the summoner's eidolon. They may find the Road Keepers or the goblins in book 1 challenging in that respect, and will definitely be challenged by the Horned Fangs and Sinspawn, given their melee propensity.
If they can survive the early levels, they should be good for the rest of the AP. 3rd level spells will help fill the gaps that are missing, and the AP is very spellcaster friendly.
I'm a bit scared about Hollow Mountain to be fair; a longer Dungeon Crawl at a level where their damage options are limited could end up a long slog; And since i don't wanna shorten the Dungeon either, i was wandering what my options could be;
Thanks for the insight about mind-affecting effect, didn't think that one out by myself!
That party looks solid. The eidolon provides a serviceable meat shield/beatstick, and all these classes are excellent choices. While you may have a little bit of problems with durability, overall this looks perfectly fine. I don't think Hollow Mountain will be a big deal; Sorcerers have plenty of spell slots, Summoners have their spell-like ability on top of their eidolon, and Witches have hexes; that's a lot of staying power for caster classes. Yeah, you'll need to stop and rest, but probably no more than any party.
Book 6 might be challenging. The first dungeon boss fight consists solely of creatures with Immunity to Magic. Not sure how this group would handle that, but for the most part they should be ok.
Some encounters will be more difficult or less difficult based on party composition; that's normal. They're supposed to be 17th level by this point, and quite frankly if they have nothing to deal with magic immunity at such a high level then they're doing something wrong. Sorcerers have like 50 spells known by this point; if literally nothing on your list is useful, that's a problem with your spell selection.