The biggest downsides of privacy tech like ZCash and Monero as that they hugely hurt scalability, 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.
Mimblelwimble is the opposite, 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 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 Grin and Monero hide van be found at
https://forum.grin.mw/t/scalability-vs-privacy-chartwhich also shows how scalable various blockchains are.
I 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 at least. It's hardly new by now.