Okay now that I've actually ran it, wew this mods a doozy.
1) The premise of the mod is amazing. Its rich in lore, the stakes are high, and the challenging is immediately appealing. Who doesn't want to storm the Thayan Embassy?
2) The writer of this mod knew what they were doing when they made this APL 10. The fights and the enemies clearly had a lot of thought put into them.
3) I like how apt the title is. This is an Embassy where Evil succeeds. The challenges and acts explicitly here punish the Good alignment in a way that fits well (imo) with the Thayan modus operandi. For all everyone likes modules where the good guys win, I think its important to have ones where it is clear that good is not easy. In a sense, if your PCs begin reconsidering their desire to help and save others as a result of this mod, then the Thayans have succeeded.
4) I love the final boss fight. Its hard, but not as hard as it seems. There's definitely a feeling of unfairness with the boss, which I think is warranted considering the setting.
1) It is -insanely- complex to run. This is both a plus and a minus, but usually more a minus. There is so much to track every round, and it slows things down immensely. I had a co-GM help with the environmental effects and we still forgot a lot of things. Players got irritated at points, because they didn't quite 'get' the Red War effects and didn't like how everything felt like a penalty.
2) This also means the mod takes really long to run. IMO, you either focus on combat, the lore or the social. You can't do all three, or even two if you want to really do one properly. I ran mine as a high-combat, high-stakes game (you are storming a Thayan Embassy, you know what you are getting into) and tried to squeeze in bits of lore about Karsus and the Battery and some dialogue with the Wizards and Malkyn and we ran to 5 hours. This was after I made modifications to streamline.
3) The mod does not reward good. This may upset some players, who enjoy feeling heroic - Embassy is honestly less about the PCs as heroes, and more about establishing the Red Wizards as villains. Some might find this compelling, with players that enjoy the challenge both mechanically and roleplay wise. Others might not find it their cup of tea.
4) Its design is very polarizing - all the random/rolling effects stacked together mean that its difficult to estimate time. Players that trigger the alarm early may end up facing grueling turns of combat, while those who are luckier or sneakier may breeze past it all to the boss. As written, its very difficult to control, and the guidance (to just 'cut' straight to the end) is rather unsatisfying.
5) On this note, I found that when running, my players would literally move between rooms as combat drew out. This felt awkward at some points, where encounters were clearly meant to be in 'isolated' scenarios, except since doors can be entered even while in combat, I had to string together scenes or 'suspend' them until the party entered.
Things I did to streamline running the mod:
1) Don't roll for rooms. You will take forever, and the time becomes impossible to estimate. Do some prerolls, lock it down to maybe 4-6 rooms max (including the key ones) and leave it there. If you intend to run combat, or your players trigger the alarm super early, cut the number of rooms and skip them to the key ones. If its exploration/social, be a lot more lenient with alarms and patrol tactics.
2) I prerolled like 100 rolls for wild magic and had my coDM adjudicate them. We went through about 60+ surges with a party of nearly-all casters (except the lone fighter). Surges take a ton of time, and some of the random effects can cause a lot of delays. Don't be afraid to just handwave an effect if you are tight on time.
3) I modified the chapel fight. Some people may disagree with this, but instead of a slow trickle of mobs, with initiative rolls and attacks - I just had the archers stand up, shoot their arrows at the button presser, then sit down again. No 'real combat', but more like a trap. To keep with the stacking effect, I had each failure compound. It made things significantly faster while keeping what I felt was the intent of the puzzle room.
4) I honestly forgot the time speed-up thing so many times I effectively ended up just cutting it entirely. It was too much with all the WM effects and other debuffs/buffs running. Maybe if I had an online platform to track rounds properly, but if running F2F there's too much book-keeping already.
Overall, I think Embassy of Evil is a very rich module - all in terms of ideas, design and complexity. Running it is definitely not something to do lightly, but done well and it can be one of the most memorable and challenging sessions your group will experience (or die trying :P).