My gpu goes unused unlike in folding@home, and CPU is throttled to 70%, but it will spike to 100% which sure enough causes trouble. I doubt this is optimal even for intended purposes since CPUs will go into overdrive increasing wattage and stuff negating the attempt at improving lifespan of hardware, and reducing noise in my case (I doubt that the fan constantly spinning up and slowing does it great service). Any chance that you could try and set the workload by cores (something I belief folding@home also supports.)

GPU: gtx 980, win 10 64bit.

Both options are available. You can set preferences in either of two different places: either in the global preferences web page of any of the projects you're attached to, or locally from the Tools menu in BOINC Manager. Pay attention to the settings you use, and be consistent: local settings override web-site settings, so you can't mix them.

With that out of the way, the ones you are interested in are at the top of the page in both places, and the (current) wording should be the same:

Use at most 100 % of the CPUs
Use at most 100 % of CPU time

Some projects may have older versions of the website code, so check the wording carefully.

It sounds as if you might have chosen the second of the options to change (throttle by time). With GPUs especially, I would set that to 100%, and change the first option (number of CPUs) instead. Note that works as an integer number of cores used: your 70% would permit 2 of 4 cores to be used, or 5 of 8 cores.

My gpu goes unused unlike in folding@home

Make sure BOINC isn't installed as a service, as then the GPU isn't used.
Other cases where the GPU isn't used:
- Incorrect driver installed.
- Windows fast user switching.
- Using Windows Remote Desktop.

Always install the driver from the manufacturer, do not let Windows do it.
When using FUS or RDP, you're using a default driver from Windows for all things video, thus the GPU will not be detected.
If in doubt, post the first 30 or so lines from a BOINC start-up log.

and CPU is throttled to 70%, but it will spike to 100%

If set via the Use at most N % of CPU time preference, then know that this one isn't a sustained throttle. It'll pause your CPU 3 times per second and run it flat out for the other 7. If you want to use a sustained low power throttle, use a third party program such as Tthrottle.

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You get the option only in advanced view. in simple view only you get useless by time option.

I'm running Boinco in VM, so like none of GPU problems should apply?

installing CUDA, or actually that doesn't work in VM.

Depends on the VM, your hardware, your software etc.
To be able to use GPU passthrough in a VM you need:
- a CPU/motherboard that's capable of GPU passthrough
- CPU may not be a K version (overclockable Intel) when Sandy Bridge, Ivy Bridge, Haswell, as these don't have VT-d.
- a GPU that's capable of GPU passthrough
- a VM that's capable
- to enable VT-d/AMD-Vi/IOMMU in the BIOS/UEFI
- run Linux as the host OS
- use KVM (Kernel Virtual Machine)
- Install Windows as the guest OS in the VM
- a second videocard for the host OS
- a second mouse/keyboard for the VM

You say you use Windows 10, well too bad, but GPU passthrough does not work on any Windows version.

This is quite a comprehensive step-by-step manual on how to accomplish the above.
Watch the video for more information.

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