We had lots of problems with MsFs 2020 ... And will continue having them for maybe a year or more And most of the problems we were/are/will be facing are not vmsAcars related, but directly caused by Asobo/Ms itself. Like assigning 5 letter codes to imaginary airports, not allowing to read scenery data, not allowing 3rd party apps to read aircraft data etc etc. As it develops, all will be solved (i hope)
SmartCARS currently have no advantages for v7 'cause it is not compatible at all. But if we imagine it works (with its current form) then main disadvantages will be;
1. Not working with v7
2. It is expensive
3. It does not offer flight save/resume option without a subscription
On the other hand, compared to SmartCARS vmsAcars does not have eye candy or ear candy features (yet) like custom sound sets being played during your flight , or a chat feature , an online radio etc and just a little bit limited maybe (this depends on the sim and addons used though, not a fact for me)
As @ProAvia mentioned, vmsAcars has the biggest advantage for v7
1. It works with v7
2. It is developed by the same person developing phpvms
3. Cheaper
4. Does not consume cpu/gpu/ram resources compared to others
So, if you want to stick with v7, then your only option is vmsAcars unless you are capable of developing your own