The USSR has just alienated the entire African Union. Great job. I am sure we will hearing the partying in Washington all the way over in Moscow.
The USSR supporting the Derg (who were pretty clearly not communists) and helping Ethiopia fight the fairly popular Somalia really upset their fellow socialist regimes. So for sure, intervening in favour of Somalia weakens their own territorial integrity and makes them unpopular with the AU, but relationships with other Socialist states, especially China and Romania, would be improved. (For both China and Romania, helping Said Barre was partly an opportunity to score nationalistic points by supporting the side fighting the Soviets, but I think a bigger motive was genuine distaste for Moscow behaving like counter-revolutionary thugs and putting their money where their mouth was.)
So while getting involved on the Somali side is a bad idea, it's not necessarily that much worse than getting involved on the Ethiopian side.
This is true, mostly because Siad canned many talented generals (honestly many of the successes of the Somali Army came when commanders ignored orders from Mogadishu) and the flow of fuel shipped in through South Yemen was cut off. Despite this, though, even with their mechanized forces having to ration their fuel the Somali Army was making shocking progress. It wouldn't be much of an exaggeration to say that the Ethiopian Army had collapsed - Mengistu was a bad commander-in-chief as well as a bad human being - and the reason for my dire predictions for Ethiopia is more based on their non-presence as a fighting force than the logistical state of Somalia's army.
Why was the flow of fuel cut off? Because the Soviets told South Yemen to cut Barre off?
I gotta say, it would be fun if Said Barre reached Addis Abbaba and helped the Ethiopian's People's Revolutionary Party and/or the All-Ethiopia Socialist Movement into power, successfully dressing his invasion as "spreading the true revolution and liberating the workers and the peasants from fascist oppressors".
I once toyed around with something like that in a TL, along with Somalia successfully convincing the leftist and nationalist enemies of the Derg to join a "horn of Africa federation". No idea if the Somalis could work with the Ethiopian anti-Derg factions so productively though.
fasquardon