Battlemechs are absolutely amazing units, yes as you pointed out they are expensive and very painful to loose in pvp but they are hands down the most powerful frontline unit in the game, in the early game or with low research they can be a bit problematic at times but by mid to late game they become almost unkillable unless your fighting deadeye’s or more powerful enemies. At max rank they get over 230,000hp 25,000 attack, 40% damage reduction, 60% additional missile damage reduction, 30% evade, cannot be stunned, frozen, burned, poisoned, or morale broken... they are expensive but so worth it! They lead me to a city level 32 clear of void 80 and I’ve assisted many other dwarves on my server do the same, Battlemechs are very powerful.
All that said Mechs and dwarves on the whole are a very expensive race to play. Without Gafgar and Jacques your really going to hurt in pvp. I very highly recommend running Gafgar with Temporay Repair and Repairman maxed if your going the mech route or you will be running troops without healing vs every other player in the game with Vega AoE healing there army for 20+ seconds which is a loosing battle or at least very expensive...
As for engineers, they continue to be the worst gold unit in the game, they are the only unit in the entire game that gets no additional skill or upgrade from silver to gold beyond more health and damage. Every single other playable race unit gets a new base skill from silver to gold rank. Now engineers do get a lot of health and damage for a small unit. By rank XIII they have 85,000hp and 43,000 attack with 134 upgrades ( 20% life and 35% damage boost remains to be researched so they should break 100,000hp and 60,000 attack ) but even still I would only ever put them on the wall as they morale break and are small so they get clobbered by large and giant enemies and with options like Bombardiers, Battlemechs, Deadeyes, and Excelsiors available they are dead last as a choice for any gold army troops imo.
As for borrowing, I assume you mean for the void issue where if you immediately return the units to the owner after entering the void that the void copies loose all the owners research. This can be avoided and you will continue to keep all of the owners academy upgrades as long as you don’t return the units until you finish your void clear so if you can find someone to help who doesn’t mind loaning units for 30-45 minutes while you finish the void you can run full power loaned troops and then send them back to the owner once your void heroes have no stamina left.