I've been using avid BB-7 road discs since 2009/10 on the commuter / tourer with no issues at all. And that included some long long descents with a very very fully laden setup - bike, rider and bags would have been well north of 150kg.

I've hydraulic on the road bike that I've had for the last 18 months , and that's nice. Possibly bit more modulation, but with decent cables and good cable management then there's nothing to stop you getting good results on cable. The key is if you are prepared for the slight aero / visual impact penalty on a TT bike from the cable runs / angles - not an issue on a commute / tourer, and a very slight issue in any case.

The difference in price between mechanical and hydro should be less than $300. Differences of $2000 can generally be attributed to other upgrades. As for differences, I've been using road discs for over 10 years from BB5/BB7/Hayes/TRP Spyre/TRP Hy/RD hybrids to shimano and sram hydros now and I'd never go back

You can get entry level bikes (<$2000) with 105 hydraulic. It's not that big of a deal.

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What's your CdA?

Oh I know. I was just wondering if there’s a big difference. I’m concerned that with a tri bike the cable bends will cause less modulation with mechanical disc much like mechanical rim. I’ll prob get a frameset and out hydro disc on whatever I buy.

I can't talk much for tri bikes but my gravel I have mechanical bb5 and my fat/mtn I have hydro treckos.

For road and gravel, I don't think hydro is necessary, nice yes but not necessary. Hydro is great for sharp braking on hairpin turns and repetitive abrupt braking on mtn biking.

With hydro if the housing gets cut and fluid leaks, your pretty much brake-less. It requires maintenance, re-bleeding etc. So factor that in if you don't have the proper tools.

Overall if you ask me, a good quality mechanical brake would be just fine for a tri bike.

I have TPR mechanical on my Felt. Wonderful better than any rim brake. They are much simpler, they are lighter, can use a standard brake lever. If you fly a bike they are much easier to deal with. TRP brakes push from both sides not like the BB5 or BB7 which push from one side.. mechanical are not self adjusting but that is not big deal. Calfee uses mechanical on his tandems and that is a lot of bike to stop going down hill. Very pleased with these brakes on a tri bike.