Sukuk
What a certificate actually entitles you to, the asset-backed and asset-based split, and the critique of the latter.
What a sukuk is
A sukuk is not a bond, though it is usually priced against one. In principle the holder owns an undivided share in an asset or a venture and receives what that asset produces — rent under an Ijara structure, a profit share under Musharaka. A special purpose vehicle holds the asset, issues the certificates and passes the income through.
The Gulf is, with Malaysia, the centre of global issuance: sovereigns, banks and infrastructure projects all fund this way, and the instrument is now a routine part of GCC public finance rather than a niche.
Asset-backed and asset-based
The distinction decides what you own. In an asset-backed sukuk the holders have real recourse to the asset: if the issuer fails, the asset is theirs to claim. In an asset-based sukuk the transfer is a legal form, and the holders' recourse is to the originator's undertaking to buy the asset back at face value — which makes the return and the risk those of a bond.
This is not a fringe objection. In 2007 Shaykh Muhammad Taqi Usmani, then chairing AAOIFI's sharia board, stated publicly that the great majority of sukuk in issue did not meet the requirements, principally because of those purchase undertakings at par; AAOIFI restricted them the following year. Most issuance since has nonetheless remained asset-based. That the question is live rather than theoretical was shown in 2017, when a UAE issuer argued in court that its own sukuk were not compliant and therefore unenforceable.