I use tons of VST beat synced plugins on my track, I have LFOTools on almost every track, also I use Effectrix, Stutter Edit and one of my griefs with Sonar Platinum is that lots of small operations (add/remove plugin, some edit operations, open or close melodyne etc.) do break the tempo and all the beat synced (not tempo synced - these ones are fine) become out of sync after, so basically you have to stop and restart to get the beats back in sync.
 
 
 

Yep this is a major gripe with Sonar with me as well. It's very easy to break the tempo during playback, even by toggling a send on or off. I've also found that it can affect plugins which are synced with song position. For instance the drum sequencer Geist has its own song sequencing mode in which you can sequence patterns along a timeline, which is in sync with Sonar's. Sometimes if I toggle a loop on or off in Sonar, Geist will become many bars out of sync until I stop and start again. 
 
I've also found that in some projects, if you're looping and you do things like insert or delete effects, it doesn't just affect the timing of beat synced plugins but it can also affect the audio as well. I had this problem in a project last night where the kick track was becoming what sounded like half a beat out of sync every time I toggled a send or inserted an effect. 

P.S. Sonar needs something like LFO Tools built in!
 

sharke
Yep this is a major gripe with Sonar with me as well. It's very easy to break the tempo during playback, even by toggling a send on or off. I've also found that it can affect plugins which are synced with song position. For instance the drum sequencer Geist has its own song sequencing mode in which you can sequence patterns along a timeline, which is in sync with Sonar's. Sometimes if I toggle a loop on or off in Sonar, Geist will become many bars out of sync until I stop and start again. 
 
I've also found that in some projects, if you're looping and you do things like insert or delete effects, it doesn't just affect the timing of beat synced plugins but it can also affect the audio as well. I had this problem in a project last night where the kick track was becoming what sounded like half a beat out of sync every time I toggled a send or inserted an effect.

Exactly.
 
I am not familiar enough with other DAWs but are they also sensitive to the addition/removal of plugins and other aux during playback?
 
Also one thing, I realize the addition or removal of a plugin or an aux track can be complicated behind the scenes (especially if there is latency compensation involved) but there must be a way when the playback resumes (even if there is a pause) to keep on sync the beat synced plugins.

Isn't this related to the concept of "gapless audio," which unfortunately Sonar's engine doesn't do?

sharke
Isn't this related to the concept of "gapless audio," which unfortunately Sonar's engine doesn't do?

As a start, I could survive without "gapless audio" if the beat synced VSTs did stay in sync. Does any of the other DAWs implement "gapless audio"?
 

I think Abelton does, and I'd be very surprised if it had time sync problems. Otherwise it would be useless "Live" as it were.

Another oddity when using beat sync'd VSTi's is if you have a loop set and the beat sync'd VSTi is the only track, even if you start playback w/ the now marker at the start of the loop, Sonar rewinds to measure 1 and starts playback from there. This is hella annoying.

+1 to this... Need to comment so I remember to vote on this one. I have the same issue with Geist that is upsetting... Can do anything you want in Geist with the transport rolling, but one wrong move when linked to SONAR throws the synch off.

I work with Ableton Live 9 (9.5 now), as well as Pro Tools and I've run into the occasional "drop out of sync" in both when loading a plugin during playback or enabling/disabling a plugin etc.

As a rule I never load instances of plugins or effects of any sort or enable/disable them during playback (especially when a plugin or proecessor is known for increasing buffer/latency due to lookahead or needing a larger buffer than playback is currently set at). 

I don't find it the least bit bothersome to simply stop and restart the playback to get things back in sync.  I will add that there are times when things just didn't seem to want to allow me to track/record with input echo in-time even with just a single ProChannel EQ enabled on a single track without causing noticeable input echo delay for monitoring through FX.  I haven't run into that situation often but it certain can be a session stopper when it happens and my clients rely on hearing themselves through FX to perform properly.

The most recent occurrence of any sort of beat-sync oddity I've been running into is during tempo changes when using Kontakt 5 with tempo-sync'd instruments and triggering at the right tempo.  In these cases I have at least been able to workaround the issue by going into the instrument settings within kontakt and forcing manual tempo to the proper tempo I need it to be at.