I happen to notice (but not fully certin sence I dont play with other people much) that people dont use envoys and pacts in the game. Are pacts really useless when in reality they can buff your fleet up massively....mainly the TEC Pacts (woot!)
You're right. In online games people rarely build Envoys, and/or get Pacts. Why? For all the reasons listed: money, time, resources, envoys (fleet supply), labs (logistic slots), research, etcetera, etc..
But I think the biggest reason is the ridiculous 12 fleet supply that each Envoy consumes. And it takes too many Envoys, or an enormous amount of time to raise relations to a point that you can actually get pacts. Especially the supply pact which you mention (8 civic labs and 17.5 relations)!
A good online feeder (eco player) only has 50 supply in excess (at the standard Normal Fleet size setting). Therefore, he can hardly afford to use about half of his remaining supply for a couple Envoys. He needs to reserve some for scouts, and colony ships for capturing neutrals, as well as his Cap ship and support. A couple Envoys will barely raise relations at all.
If you increase fleet capacity you suffer increased fleet tax (+9.5%) on all income, and subsequent feed is also taxed at the higher rate.
The researchable techs for Envoys are probably a better deal than pacts, but nobody seems to use them either. In particular, the tier 4 Vasari tech that essentially doubles a planet's extractor output and it's factorys' ship production rate while an envoy is present, is quite valuable and applicable to almost all planets. The tier 3 TEC tech that increases the envoy's planet's trade by 50% 1/3 of the time can be very valuable, especially if it is a desert loaded with tradeports, but rarely will a military focused teammate have that type of planet present. Also, the tier 2 TEC "Worthy Cause" tech that 'donates' $60 every 30 seconds is nice and usually worthy. Advent's tier 3 envoy culture tech is crap, IMO, and also rarely applicable. All of the remaining higher tier Envoy techs are also all generally poor investments, and/or very situational.
IMO, Envoys should kick back something (maybe a portion) back to the owner as well, (beside a measly .10 relations boost with slow relations growth). Also, I would like to see Envoys having some form of small military value.