Yes, it is less secure, in fact, you technically do not own the bitcoin since you are giving the "control" to them. Both apply for verified and unverified.
But I won't call them giving a false sense of security, they just offering a custodial service.
Yes, Blockchain dot com generates an industry-standard 12-word recovery phrase. As long as you keep the phrase secure, you able to simply recover it by the phrases on many wallets. The compatibility is ensured since it is being used widely.