I'm doing my first topsy turvy cake following a very good tutorial here. Unfortuneatly I feel a cakastrophy coming. My niece didn't want fondant, but I feel like the only way that this cake is gonna look good and stay together is to use fondant instead of the buttercreme. Does anyone have any experience making this in buttercreme and do you feel that given the heat (gonna be 90 degrees) and the construction, I should take my chances with the buttercreme or is the fondant a better idea?

AI've never personally done a topsy turvy cake...but I've never seen one done in all BC. If she's not wanting fondant because of the taste there are some you can make that taste pretty good if you have the time to make it. Or if you even want to! Maybe you could explain to her your concerns about it not looking right if you don't use fondant?

ATupsy turvy, BC and 90 degree weather. Total no-brainer. Your niece can accept the medium that would make her design possible or choose a different design or a different baker.

AThe only way I would do a topsy turvy... Even in the cool British climate... Would be to use ganache and fondant. I wouldn't even consider buttercream and fondant. In UK summertime, which is no where near US or Aus temps I still had a tier buckle when using ganache and fondant.

I have done a few topsy turvy cakes in buttercream . I used Sharon Zambito's method and it is tricky but doable

i wouldn't do one unless it was ganache under fondant.  I don't do them enough to feel comfortable enough to do them in buttercream. 

Like it has been said...you are the professional with the knowledge about construction and your abilities. Your niece needs to understand that there are limitations because of climate and materials. 

I recently did a topsy turvy in BC and it turned out fine. I used a hi ratio shortening and butter mix in my BC and used Sharon Zambito's method also. It just took a little more time to smooth because of the angles. I just had to make sure to keep refrigerated until delivery and in the AC at the venue.

This was my first topsy turvy and it drove me nuts because I am a stickler for "level and straight" lines on a cake- my husband laughed at me the whole time I was building it because he could see that I just wanted to make it straight

i did a regular cake last year on a 90 degree day, and it was in the bride's air conditioned house until well after dark and that thing fall apart, inch by inch, crack by crack, until I had to beg the bride to cut it before it collapsed altogether. 

something to consider

if i were doing it again, I would have made a small tiered cake that could have stayed in the refrigerator until an hour before the cutting and had sheet cakes on hand for the rest. 

I do them all the time. Use a firm crusting buttercream with swetex and you will be fine.