The -50% doesn´t necessarily mean half combat strength.
Civ 5 has a big inconsistency problem with percentage boosts, they are added together before actually being implemented, I think.
In case of building boosts that makes sense and also keeps a lot of things from getting too overpowered, imagine a City with University (and Rationalism), Research Lab, National College and Observatory, each plus 50% for a total of 200%, equaling 3x Science versus that being multiplicative 1.5^4 ~ 5.06x Science, anyone not getting a mountain would be scientifically screwed compared to other civs that have one
However this seemingly also applies to combat strength, which often doesn´t make things as worse as losing half the combat strength, I think that plus 50% and -50% cancel each other out, instead of halving your strength and then adding 50% of that (equaling 75% of originial combat strength).
I´m not 100% sure, but if that applies, in a lot of situations, this will be just a minor inconvenience, for example Rocket Artillery (requires Aluminium, without promotions) attacks City: Because it is a siege unit, +200%, then -50% means +150% remains, which just reduces your final multiplier for your Rocket Artillery from 3x down to 2.5x.
Now in case of a Rocket Artillery defending (even against cities, that 200% only counts on offense), that would be halved, but the RA can stand outside the city and if you can protect your RA, you have a very minor downside.