Yes that's true. It's better to have the same attributes (e.g. fire x5, or lightning x5 & etc.) and same tier grade so it will result probably in an upgrade than a downgrade. The number of swords combined may differ in results. In one old post I have seen a player combined 2 tier 15 legendary swords and resulted in a berserker sword. Also my friend Kartik if I remember combined 10 to 12 swords and resulted in a Dark sword. See the difference?
Remember that it is not a guarantee that you will get one instantly. It's always a gamble but here you have a chance to get a unique sword. Luck must be on your side and surely it's a great blessing if you do get one 😁