Can I bypass crontab command by putting the file?

I need to create and put my cron file with

cat cronfile1 cronfile2 > cronfile
crontab cronfile -u myuser

This works for me, but I want to know if I can do directly

cat cronfile1 cronfile2 > /var/spool/cron/crontabs/myuser

Of course I am root, and I’m doing this from busybox. Is it safe or am I doing something wrong and should use the crontab command instead?