Ranked in these QuestionsQuestion Ranking
Pro Really platform-free
Pro Doesn't store passwords
If it's not stored, it can't be stolen.
Pro Open source
Con Easily cracked with exposed/compromised master password
Since anyone can use LessPass with the same parameters as you, all they need is your master password (which could be obtained through keylogging or numerous other methods) to gain access to every one of your site passwords.
Con Cannot store existing secrets
Existing secrets (aka passwords) cannot be stored with deterministic password managers. In order to use LessPass effectively, you will need to change your password on every site.
Deterministic password managers have many drawbacks.