The EX5/7 did have FM, but it was 2-op and buried in the AN model. The SY99 (and SY/TG77) was Yamaha’s last big flagship FM workstation before the MODX. I suspect that Yamaha expected FM enthusiasts to buy the half-baked FS1r if they wanted more FM. The failure of the FS1r (and success of the Motif line) likely discouraged Yamaha from investing in FM products until recently.
If you can find a good deal on one, I strongly recommend the EX5 and TG77. The SY77/99 are big, heavy and even harder to find than the EX5 is, and the TG77 is only missing the sequencer and the SY99’s sampling option. The sampling option requires a memory upgrade that is likely even harder to source. (also, I strongly recommend against the FS1r. Get a OPSIX or a MODX instead)
