These are my top unused tech mostly due to athe AI not using them and in some cases also as they do not make sence to be used from player side:
1. Ion Guns (both batteries and PD): Less effective then modded laser, turbo laser and in some cases also other options like coin/railgun,. Specialised tech such as disruptor also has more damage output + special effects per default
2. Asteroid Base: Mentioend this already a couple of times. Due to how tactical combat works, increased HP is irrelevant ofr a non-combat base, and the amount and type of slots prevent it from configuring it as a usefull combat base. The total amount of planetary base slots is limited and this can also only be built if planetary pre-requiremtn is met so basically is never researched and built. Suggest giving it harpoints to be able ot function as a cheaper long range version of starbases able to host heavy guns (the range of which would even be longer then that of planetary defneses) and making this an excellent defense option - if the AI would use it then it would give it an edge and make siege of these planets more interesting
3. Boarding Module (+Marine Quarters): A classical exploit as it is cheap/early tech that cannot be countered so reffraining myself from using it at all as it is really not fair against the AI. I would either make it a late-mid tier tech, give it to pirates and include it in some AI templates
4. Habitat Module: This one is inefficient as I find the planetry pop capacity is almos never a limiting issue, the slot it takes up can be used well for a shield though.
Shield in particular are so increadably usefull to have (due to regen rate) that they push out all other alternatives most of the tim inlcuding bulkheads and range extension. There is also a mod for armor penetration but not agaisnt shields which makes armor suffer even more under the clear benefit of a regenerating shield barrier
If the DLC update really addresses some of the techs, maybe some of the above could be tackled