The biggest downsides of privacy tech like ZCash and Monero is that they hugely hurt scalability, not just by having much larger transactions, but also by making it impossible to identify the UTXO set. Because you never know when outputs are spent, you have to maintain the entire TXO set (i.e. not only store but be able to efficiently index it) all the time. (When Monero fans claim that it improves scalability over Bitcoin, they conveniently ignore these properties and instead refer to Monero's ability to increase the maximum block size under conditions of congestion.)
Mimblewimble is the opposite, allowing you to completely forget about spent outputs, even in the Initial Block Download, greatly improving scalability and privacy at the same time.
A much more objective comparison can be found at
https://phyro.github.io/grinvestigation/why_grin.htmlThe one downside to Mimblewimble compared to bitcoin, is that it no longer allows full auditability.
But at least in Grin, auditability reduces to one simple equation. Quoting from
https://np.reddit.com/r/CryptoTechnology/comments/kyhgcv/are_there_any_public_cryptocurrencyblockchainΣ utxo = Σ kernel + offset * G + height * 60e9 * H
Another feature, that can be considered both an advantage in some cases, and a disadvantage in others, is that MW transactions are multisig by sender AND receiver, and thus require them to interact to build the tx, just as is already the case for Lightning. The advantage being that you cannot receive unwanted coins (like tainted ones), and don't need to scan the blockchain for new outputs unless you just transacted. The disadvantage is that you need to be in communication with the recipient.
Note that Litecoin's MWEB implementation is not pure MW, but a more complicated hybrid that no longer requires receiver interaction.
The main difference I noticed was grin being considered fairly weak for privacy as it hides historic information and transaction amounts but those can be gathered before a transaction is confirmed
This is quite wrong. An accurate overview of what various blockchains hide (and how scalable they are) can be found at
https://forum.grin.mw/t/scalability-vs-privacy-chartI THINK I'd add an con of the grin community being new and grin coin being fairly new too - I think that's their biggest drawback so far (just the newness, nothing to do with the people).
Grin has had a running testnet since 2017. It's hardly new by now.