Please share your address here or the exact Error you're getting. Afaik for every failed transaction, a specific Error is stated:
ex:
Warning! Error Encounter during Contract Execution [Bad jump destination]
Warning! Error Encounter during Contract Execution [Out of gas]
And is it still safe to use mew to store all your tokens from airdrops and bounties?
For me, yes, as long as your Private Key has not been exposed to others. Btw, they have
MEWConnect app now for extra security
(available on iOS, they're still working on android version)