I am doing spesh, physics and umep maths this year. I can't say anything specific about umep physics and umep indo but I can talk about spesh and umep.
Spesh:
Dropping spesh won't stop you from doing anything that I know, except the accelerated first year uni subject that has a pre-requisite on spesh score. I actually quite like the content. Personally, I think if you like the content, doing spesh is not a bad idea because they will give you better knowledge when you go proceed to your engineering degree. For example, vector in spesh gives you some knowledge in linear algebra, which is a level 1 uni maths course. Maths in engineering degree will be harder than spesh anyways.
Workload-wise, I think it depends. If you understand the content instead of memorising ways to do a question, it might not take that much effort.
But next year's spesh will include a bunch of new content, so I'm not sure whether vcaa will do something crazy. This is also something to think about.
UMEP:
Absolutely love it. I learn content at uni level and get almost everything that other unimelb student gets. I even get a student card!! I can go to unimelb library after closing time till midnight (not that I will stay that late) to study or join student clubs. I also get to know year 12 students that love maths during umep, so I think this is a very great experience in year 12.
Only relevant if you are thinking about unimelb, but for umep physics, I heard that you can't really go straight into year 2 physics in your first year in unimelb because the maths pre-requisite is not satisfied. So you might need to do nothing physics in your first year.
Also, one thing to consider is atar. If you choose spesh and do well, it might be one of your best 4 and count 100% towards your star. This would be great considering the scaling. On the other hand, doing umep can only get you at most 5 marks because it can only be one of your bottom 2.
IMO, umep is great for its experience and uni level content, but spesh is great for its score and maths foundation.