2 sentences

1. 昂贵的东西未必质量就好。

   áng​guì de dōng​xi wèi​bì zhì​liàng jiù hǎo.

   Expensive items are not necessarily of high quality.

   Source: Dong Chinese Dictionary

2. 你的计划听起来很有道理，但在实际操作中未必行得通。

   nǐ de jì​huà tīng qi​lai hěn yǒu dào​li, dàn zài shí​jì cāo​zuò zhōng wèi​bì xíng de tōng.

   Your plan sounds reasonable, but it might not work in practice.

   Source: Dong Chinese Dictionary