This depends on what source you want to use for Atmos. Apps on your TV will use compressed Atmos via DD+ For which HDMI-ARC is enough. If you want to get uncompressed Atmos from a BD-player, you need eARC.

You asked for opinions, so here goes.

1. No. If a TV set has HDMI 2.1, it is supposed to have implemented the CEC specifications properly for ARC and eARC. Unfortunately, some manufacturers ‘extend’ the capabilities so that their own brand of speakers can do ‘more’, often at the expense, and perhaps unwittingly, of impacting the actual CEC specifications. Sonos, and frankly pretty much any other ARC/eARC client device out there, has implemented the CEC spec as written, and likely doesn’t have the wherewithal to purchase and test every of the thousands of TVs out there, since any single TV from a manufacturer may use different hardware and software from another TV from the same manufacturer. 
    
2. Atmos is a type of data, that can be encoded in multiple types of carriers. One, which is considered ‘lossy’, because it limits the number of channels (7 instead of 20+ rings a faint bell, but I haven’t looked at the specs recently) is Dolby Digital Plus, which is carried across ARC. Other types of Atmos (TrueHD, MAT) are too data rich, due in part to the number of channels, to be carried across ARC, and require eARC for the appropriate bandwidth required. Using the Beam Gen 2, Atmos via ARC using Dolby Digital Plus should be fine, since the Beam doesn’t have upward firing speakers like the Arc does, and really doesn’t need the extra speaker channels provided by the eARC required versions of Atmos.