Definitely check your target user base, the biggest split I’ve seen is 67/33% iOS/Android in US healthcare and most common 50/50% for the mainly US enterprise users I work with.
Navigation and menu paradigms are very different and will probably have the biggest effect on any design you do, it would be worth reading up on the differences.
The native controls are much different between iOS and Android, so you may want to factor that in. Especially pickers and list controls. Most of the time this hasn’t impacted my designs drastically since the functionality is mostly the same.
The other potential gotchas are functionality, if you’re strictly working on the visual design you shouldn’t have to worry about this. However, if you’re helping problem solve functionality and the user experience, there’s definitely differences that can make or break feature parity. Each OS handles notifications, alerts, service integration differently. Going through any early workflows with your app developers should help here to spot any blockers or “expensive” designs.