And while we made it work by sticking to the least common denominator which was FaaS/IaaS (Lambda, S3, API GW, K8s). It was certainly not easy. We also ignored tools that could've helped us greatly only against a single cloud in order to be multi cloud.
The conclusion after 2 years for us is kind of not that exciting.
[1] AWS is the most mature one, Azure is best suited for Microsoft products and Old Enterprise features. And IBM is best if you use only K8s.
[2] Each cloud has a lot of unique closed code features that are amazing for certain use cases ( Such as Athena for S3 in AWS or Cloud Run in GCP). But relaying on them means you are trapped in that cloud. Looking back, Athena could have simplified our specific solution if we were only on AWS.
[3] Moving between clouds, given shared features, is possible, but is definitely not a couple clicks or couple of Jenkins jobs away. Moving between clouds is a full time job. Finding how to do that little VM thing you did in AWS, now in Azure, will take time and learning. And moving between AWS IAM and Azure AD permission? time, time and time.
[4] Could we have known which cloud is best for us before? No. Only after developing our product we know exactly which cloud would offer us the most suited set of features. Not to mention different credits and discount we got as a startup.
Hope this helps.