It does have a solid idea but the adventure really bogs down in the combat. For us it was a slog that would have TPKed the party with ease if played as intended and even when the spellcasters did not make optimal choices they still could have easily slaughtered the entire party.
The RP sections are interesting but also felt a bit inconsequential. It barely had any impact on what you do for most of the adventure and did not weigh up against the hit back and forth damage of the first combat. And that was after the flameskul ran off due to a turn undead. Admittedly the session we ran had quite a bit of bad rolls from the players but the combat feels really unsuited for party compositions that do not follow the traditional setup. Which occurs fairly frequently with smaller tables and varying amounts of players.
The battles really needed to be powergamed to stand a chance. If you don't act on player knowledge of what monsters do (Regen and what stops it? Do you have a player trained in religion? The Flameskull discourages using fire of your own and you might not have a paladin or cleric on hand.) How these monsters they work they can easily take down a low level party. You are dealing with a lot of immunities, regen that needs special damage, and plenty of AoE spellcasters.
Add to that that the balancing seems off. Remember that level 5 and 6 character parties could be running this adventure and then Jaru on its own would already be quite the challenge for a party. Add in the other monsters and environmental hazards and potentially 2 fireballs in the first round while you enter from a 5ft corridor. Going by this it feels like it is assumed you have at least one spellcaster with counterspell and even then your caster can just counter the counterspell.
I don't mind encounters stacked against the players but at least give them alternative methods to deal with the threats or to disrupt what is going on. In this case you have pretty much an undead ambush and final boss chamber you only have one entrance too and the boss seems to already know you are coming. Risk is good, tension being high should be a thing. But it should not feel like you only live by the grace of the GM.