It's not that it's a problem, it's that it's easier when both are iOS.
-many purchased apps work on both iPad and iPhone. You have to purchase separate apps for Android.
-wifi passwords entered on one device don't have to be reentered on the other.
-iMessaging
-same cables
-find my iPhone/iPad
-etc.
Obviously those are hardly insurmountable problems. Put taken altogether it is easier to stay within the ecosystem.
It depends on your usage and expectations. I don't purchase many Apps from the App Store...mostly use the free ones.
Actually I don't purchase many Google Play apps either but more than I do from Apple. I carry my phone all the time so everything I need is on my phone.
-WiFi passwords being entered into two devices is not a biggie for me.
-iMessaging....I have no issues getting messages from my family on Android.. FaceTime is a different story but my work phone is an iPhone and I can use that.
-same cables I have no work around for but not an issue for me as I carry both for work.
-find my iPhone/iPad I have an app that can find them on Android but I never lose my phones....in use it to track my daughter's phone.
In the end it's up to you. The biggest gripe I have with IOS is lack of customization and the way Itunes handles music. I tried to go iPhone but I just can't get myself to do it