My issue relates to how they're treated with respect to taxation by the various revenue authorities. In the UK, the tax paid on RSU's vesting is related to the personal income tax bracket the individual falls into. Which means for anyone in the UK's higher tax bracket, you end up losing ~50% of the amount vesting as tax.
If these behaved like other forms of equity, ordinary shares, preference shares for instance, they'd be taxed as capital, the gains of which would be taxed 'when you sell them' and at a much lower rate of tax - ~50% lower for a UK higher rate tax payer.
I'd rather a company just pay me a higher basic salary than award RSU's. At least then it's both pensionable and often used as a multiplier at bonus/salary raise calculation time.