First of all i am assuming that you have at least half your team generals. Reg is pretty good vs assiassins without priests.
Having to retreat 1000 hp earlier is more than just annoying. I means you have to leave a lane earlier. That kind of thing can cost you the game. It's also more than just 1000 hp. It's 1000 hp every 12-15 seconds (10 on cataract), delivered to you with impunity on his part. If you are holding a lane for 20 seconds, you could be down 1500-1800 hp, which is a lot. His allies will probably push you out.
Snipe damage can bearly out damage monks regen and fails vs bishops so damage over time is of minimal effect. And compared to vsing another UB, Oak, sedna, rook or LE this is just annoying, at best. Though you are behind on warscore tower camping is perfectly valid and you will not die doing this, resalting in reg being really underpowered late game.
Reg can deal a lot of damage in direct combat as well. He could be standing on 6 mines, and ready to cast 3 more as you approach. With the mark on you, you're going to take an unacceptable amount of damage before the fight even starts unless you tank those mines with a minion or oak shield. 4050 mine damage, 400-800 mark damage, and a snipe as you approach for good measure for another 700. SoR lets him recast mines in 6-8 seconds and mark you every 9-10. His autoattack isn't too shabby as he levels up too.
You assume that the attacking demigod only has monks as minions. If reg is on the other team then one of your generals is able to go a morel or just minions build (these have considerbly more dps but less utility), when minions run into mines mines will go off resaluing in mines throws being the only viable from of damage to the demigod (who can doge this, just run out from where you are), mark will be either activated under your own towers though some kind of non targeted abilty and the damage will be fixed up by monks and his dps is pretty poor once you factor in his complete lack of incombat burst damage.
And from my experence reg can't deal with spirts to save him self i have forced regs to retreat while getting my mana back at the base crystal.
Reg is only half arse powerful if your opponants play like they are versing any other demigod in the game, reg has special needs.