It appears that the STO season 11.5 update (which came out 4/12) will not run under Crossover 15.0 -- it hangs at a partially-completed character select screen. Updating to Crossover 15.1 fixed this issue.
The Crossover 15.1 update also appears to have improved compatibility with STO in two other ways:
-- A crash/hang that frequently happened near the end of the black "Cryptic" loading screen's progress bar (i.e. just before the current season's splash screen shows up) seems to have gone away.
-- An annoying visual glitch that frequently displayed 1-frame flashes of various glyph and font arrays during screen updates has vanished.
Problems that remain (with some workarounds):
-- The STO client still experiences fairly frequent crashes when zoning into high-population "social" maps like ESD, Starfleet Academy, New Romulus, and sector space. I think these are also afflicting people running directly on Windows, but haven't really tried to investigate. WORKAROUND: Restart the client from the Arc launcher.
-- The main client ("GameClient") frequently crashes during startup (half the time or more), often right after launch (with a black screen, but before the "Cryptic" logo appears), and much less commonly at the season's splash screen. I suspect race conditions in the code, since there doesn't seem to be any consistency to the crashing behavior -- freshly-started Crossover bottles running the Arc launcher and then STO will exhibit varying behavior. One thing seems to make it worse: having iTunes running. Shutting iTunes down seems to significantly increase the chance of successful STO client startup. This problem was present with the official Mac STO app as well, so it may be either a general STO client problem, or at least a generic problem running it under emulation. WORKAROUND: Fixing this usually requires killing the client with Apple's Force Quit window, and restarting repeatedly until things work, or get messed up in worse ways that require additional workarounds (see following item).
-- When the game updates to a new version via patches and launches the new version for the first time, it hangs for a long time at the season splash screen (currently "New Dawn"). It is apparently doing something computationally intensive for a long time (the fans on my Macbook ramp up to full). After 5-10 minutes, it will give an "Account Ticket Authorization Timeout" error and offer to log in again. WORKAROUND: Wait for the error message and log in again; the game will immediately give the character selection screen and work normally. Subsequent startups don't exhibit this problem until another patch happens.
-- STO client crashes sometimes leave various wine processes around that don't get cleaned up properly, which can interfere with restarting the STO client after a crash. Sometimes it also interferes with restarting the Arc launcher, with the symptom that you can't type into the password field. Quitting Crossover and restarting it doesn't usually fix this problem for some reason -- it appears to have lost track of the processes and doesn't kill them on shutdown. WORKAROUND: Use the Apple utility app Activity Monitor to kill any process named "wineloader" or "wine server". (This assumes you're not running other Windows applications under the emulator at the same time, so beware!). Click on the "name" column header to reverse-sort the list so the "wine..." processes are conveniently shown together near the top. Start the Arc launcher again after you kill them.
-- There's a problem with the gamma setting not being properly restored when switching away from the STO app to any regular Mac window, and then returning (at least in full-screen mode). When you return to STO, the gamma setting appears to be reset to 1.0, even though the game's preference for "brightness" shows it set to something else (I use 1.35). This problem was present in the old, official Mac STO client, and also affects Guild Wars 2, which uses the Transgaming Cider emulator, so I suspect this is an issue with how the wine emulators handle gamma during focus changes between Windows-emulated windows and Mac windows. WORKAROUND: Trying to adjust the in-game brightness slider in the options window will cause the brightness to jump to the correct value.