At the yard I work for here on the Cape we've installed a lot of SEI lower units/outdrives , for our more "price sensitive" customers.
They seem well built, and never had a problem with installation.
Anecdote 1: Just this week we had an SEI Johnson replacement lower unit blowup on a 22' SeaWolf (Seacraft knockoff) after 8 years of hard use!
Not bad for 900 bucks. Slapped another SEI unit on and he was off to the races.
Anecdote 2: Last year we ordered an SEI/Merc outdrive for a DIY customer, he took delivery of the drive, then came back a few days later saying the gear oil drain plug was stripped out, and the plug had been threaded in crooked.
Highly dubious considering these units show up brand new, freshly painted and beautiful, but I guess plausible.
Anyway, SEI swapped it out no extra charge, no questions asked.
That kind of customer service is impressive and rare.