You are not limited to 120 units. You can take as many units while fulfilling all requirements and complete in 8 semesters. I posted elsewhere that my S22 is already done with all his requirements (well above 120 units) and needs just 1 non EECs course to graduate. So he is skipping next semester and then will graduate next spring with close to 135 units.
So far he has taken 7 EECS upper divs (beyond the 5 required) and if he stays in college next sem will end up taking close to 12 upper divs.
The advantage is simply to get these breadth requirements out of the way, freeing up space in your schedule for studying whatever you want. That might mean additional upper division EECS classesā¦ or other classes such as economics (which you mentioned), math, physics, other engineering disciplines, or whatever may be of interestā¦ Cal has a wide variety of really great classes and professors! 
As @ucscuuw said, there is no limit on how many units a student in COE can take within the 8 semesters.
Thank you everyone for help in constructing the college list and advise throughout the college application and evaluation process. Since we were applying to CS related majors with lower gpa and average ecās we constructed the list very cautiously with both in-state and OOS options. In the end, we were fortunate to have many options and honestly not all options were fully evaluated. S25 focused on two finalists UIUC and UC Berkeley and after visiting both has decided.
Happy to report that S25 has committed to study EECS at UC Berkeley. In the end the choice was close as all along he wanted to go OOS for studies and not so close to home. But he wanted to study both EE and CS and the flexibility of the EECS major at Berkeley was the deciding factor and he did not want to navigate two different colleges at UIUC and other schools.
Go Bears!
Congrats - and thanks for sharing your journey - and for contributing your knowledge on other threads.
Also thanks to @tamagotchi and @ucscuuw for answering specific questions about the EECS program and helping us arrive us at the decision. Will PM you both closer to fall and would like to have your kids mentor my son if they have bandwidth.
S25 officially done with HS. Some tidbits I learned from the recent events at his HS.
UCLA: 90% of the admits at his school had one B or none. Seems to correlate well with the high GPA UCLA has in their UG admission profile.
UCB: The admits to Berkeley have a much wider GPA distribution indicating a much more holistic review than UCLA (including S25 above).
Very few were admitted to both schools. Among those pre-meds chose UCLA and business students picked UCB. Donāt know of any engineering admits to both.
Off topic. Has anybody used the expedited in person passport service? S25 wants to travel abroad for 2 weeks, but has no passport. Basically wants to book the tickets and use it to get a passport within days.
I have done it in SF in the past, no problem. But then when I had to get my Dās passport renewed in a hurry a couple years ago, we had to fly to the Seattle passport office because there were no appointments available anywhere in CA. So maybe donāt leave it until the very last moment. 
Thanks. S25 has made an appointment. Should have paid more attention to his supplemental essays. One of the schools(USC?) asked about his dream trip and now I see the country and the trip described in detail there!
Hopefully there are no requirements to be 18 where he is traveling. Just a couple of months we were worried about him checking into hotels when visiting UIUC.
There probably will be for hotels in most countries. May be ok to stay in youth hostels, but in some countries hostels also have a minimum age of 18 for unaccompanied minors.
My twins went to Europe on their own at 17. Stayed in a couple of hostels in Italy and a VRBO-like apartment in France (found on a website and contracted with the owner). Mostly Europe doesnāt seem to care outside of big hotel chains, especially if your kid has a credit card. No one ever asked their age (even in bars!).
Re no+one asking age - some European countries require hotels to see your passport, so it would be there.
Re bars, most European countries are 18 to drink but some are 16 or 17 for wine or beer. But yeah, unless youāre obviously very young, I donāt think they really ask for ID.
Edit: no idea where OP son is going so Europe discussion may or may not be relevant.
Re hostels, maybe also differs depending on whether they are HI affiliated or independent. I remember vast differences in what was applied between them in my younger days (I stayed in countless hostels in a number of countries). If heās traveling on his own (or even with friends) highly suggest hostels as a great way to meet people.
Thanks everybody for responding. S25ās friends are 18+ (but he is not) and they will be visiting Japan, should have clarified that. S25 does not have credit card but told me he will be using some kind of prepaid card for trains and apple pay for airbnb stays, maybe there is some other kind of card for other payments.
If I were you Iād at least get him a credit card (linked to your account) for emergencies, just in case he meets a large unexpected expense. He can then use the card for school expenses (textbooks, whatever) at college and start building up a credit record while heās at college.
My daughter said Japan has an ATM type card that you can use everywhere (like the Oyster card in England). You load it and then just swipe rather than using your ATM all the time. She really liked it, especially for small items and transportation.
I recently went to Spain and used my credit card everywhere āuntil the last day when it was hacked and I had to use my alternative card. I received a notice from my credit card issuer asking if Iād purchased a Tesla for $17,100. No, I had not done that. I hadnāt used my card anywhere sketchy (some ubers/taxis, at the subway and train stations). I think it might have been the vending machine in the hotel lobby when I bought water. I was leaving the next day so no big deal and I could have lived without another credit card (I did also have my debit card but that charges me an international fee so I donāt like to use it).
Suica card. There is a tourist version. They are sometimes hard to find physically (or were when we were there last year), but you can get an electronic version in your Apple wallet (which also makes it really easy to reload as you donāt have to do it physically).
I love the Suica card and the little Suica penguin! I brought a wallet with the penguin on it back from Japan and my high schooler has co-opted it as his wallet 
Thanks again. S25 was able to get an appointment using his tickets and was issued the passport the same day. Also the suica card, I believe is available on the app store and he was able to use apple pay to load it. He is having fun in Japan now with his friends.
After going through orientation and meeting with advisor S25 has unofficially 26+ credits (Reading & Comprehension A and B, Math Calculus 1/2/3 (MATH 51-53), Humanities/SS LDIV) going in with more possible when all dual enrollment courses are processed by December.
Given that he wants to spend 4 years at Berkeley what are your thoughts on these options below (especially in the context of AI disrupting the industry).
Best option IMHO. There is such an amazing wide range of great courses in EECS and beyond. I guarantee your son will find a lot of stuff he wants to take, above and beyond the prescribed EECS curriculum (which is frankly pretty minimal in terms of number of courses).
