* node resources scale with the size of the UTXO set (unspent outputs), which in Monero's case balloons to the entire TXO set (all outputs, orders of magnitude larger)
* a typical 2-input 2-output transaction is 4 times larger
* wallets have to track all outputs to choose random decoys for transaction inputs
One can argue that this is the price to pay for significantly better privacy, but the largest benefits come from having no visible amounts or addresses, which can be achieved with significantly better scalability than BTC [1].
[1] https://forum.grin.mw/t/scalability-vs-privacy-chart/8114