First, you can further split the backup. Take the following example:

You have a shamir arrangement that requires 4/7 shards to reassemble. You have 5 family members, and consider them together to be an important part of the holding, but you don't want to worry about colluding. So instead of having family hold onto 5 of the 7 shards, you split one of the original seven shards into 5 sub-shards in a 3/5 arrangement. Family collusion can no longer risk the entire account.

Second, which is hinted at by the first, you can really spread shards around without risking the full account. Making a 3/5 multi-sig can be difficult. Making an 15/20 shamir is comparably quite easy.

Third, shamir shards are encoded in a manner that avoids confusion. If you have more than 1 multi-sig, you have to track which seeds are attached to which account. Shamir shards can be identified by the first 3 words, which are shared across all shards. Each shard also has checksum built in.

Fourth, on reassembly, the result is far easier to use. Each transaction with a multi-sig requires that you once again multi-sign. Reassemble a shamir secret, and you can run whatever amount of transactions need to be run, then factory reset the hardware wallet.

Even for the purposes of long term vault storage with trustees, shamir is superior.