免不了是表主观意愿，程度比不可避免要低。不可避免表客观意愿，强调被迫性与必然性。 比如：旅途遥远，免不了路上要为自己加减衣服。 Journey is so far ,it's unavoidable to add or subtract clothes for you self. 世界上有如此多的车流，出一些车祸是再所难免的。 There are so many road and cars in our world,it's unavoidable that some car hit a another car.

There are slight differences. "不可避免“ is more formal, so you should use this more in writing,while "免不了” is usually used in speaking. For example: 战争是不可避免的。 War is unavoidable, 夏天免不了要开空调。 It is inevitable to use air conditioner in summer. And sometimes, they are exactly the same, you can use both in some context.

感觉其实差不多