That's what I didn't get. They offer a card with up to 28 ports, but say it scales up to 256 ports.

So can you daisy chain these cards, or are they going to be offering the mother of all RAID cards, with 256 ports?

I just ordered a 16 channel Highpoint SATA RAID controller for $412.

They probably mean the 5085, with 2x SFF-8088 (external x4 multilane SAS). That's two ports, 128 SAS addresses each, or even more with "fanout" expanders (see e.g. the SAS JBOD enclosures by AXUS).

Each ML SAS port can be connected to a daisy-chain or a tree of cascaded JBOD enclosures.

The internal SFF-8087 can also be used for cascading, provided that you have an expander-based SAS backplane in your server that provides an external SAS expansion port for daisy-chaining.

Those 256 drives may as well mean a firmware-side limitation, rather than the max.number of SAS addresses theoretically possible per a daisy-chain / cascaded tree. Still I'd be a little cautious about 256 drives per RAID. There can be real-world glitches that may limit the practically useful degree of cascading, performance with so many drives, choice of RAID level, maximum block device size that your OS can actually take, runtime reliability of such a monster etc.

I also keep hearing rumours of SATA drives being incompatible with some SAS expanders, or that you can only use a single expander per ML SAS port for SATA drives (no JBOD daisy-chaining) etc.

I believe the limit of 256 drives is the same with the older 3xxx series.

The 5085 is on par with an Areca ARC-1680x: same IOP CPU, same number of ports. As far as firmware features and comfort are concerned, nowadays I'd probably opt for the Areca.