I was wondering if hydrangea had any effects on the growth of other plants. Doing a google search only told me that hydrangeas are immune to juglone. However I'm more interested in the effects of hydrangea on other plants, will growing hydrangea have an effect on plants such as tomatoes, cucumbers, carrots, or peppers? I know that hydrangea has cyanogenic glycosides, and I was hoping that this would help prevent pests, but I'm worried that it may also negatively impact the main plants.
Hydrangea varieties are not known to have allelopathic properties, and their presence nearby should therefore not cause any problems to the plants you mention, nor any others, although their need for water and nutrients might deprive anything smaller growing nearby.
However, you mention cyanogenic glycosides in relation to Hydrangea; as you rightly say, these plants do contain cyanogenic glycosides, which makes them toxic to horses, cats, dogs, etc., if consumed. In fact, though, insects may actually seek out plants containing various toxic elements (including cyanogenic glycosides) in order to store them for defensive purposes, along with a possible role in reproduction, see here https://link.springer.com/article/10.1007/s00049-009-0018-6 for further research if you wish. This essentially means that Hydrangea is highly unlikely to act as a deterrent to insect pests which might affect your growing crops.
Hydrangea somehow evolved to be resistant to Juglone that the walnut produces to discourage competition by its own babies and many other plants. Hydrangea learned to be able to grow under walnuts, in the shade and in thin soil. The trick for planting plants is grouping like needs with like needs. Hydrangea has no 'trick' chemicals to protect other plants from insects. Or even itself, that I know about. Hydrangea has few insect problems but no special chemical for extra protection from insects. Hydrangea found a great niche and took advantage of it.
Don't expect lots of flowers in deep shade.
Hydrangea's needs are acidic soils, partial shade, protection within its micro environment, few competitors such as weeds, shallow roots that should not be mulched more than an inch in depth. Great drainage. Fertilizer with lower nitrogen than phosphorous and potassium in percentages. Plants that grow well with hydrangea are shade loving acid soil loving plants that also need great drainage. Plants like Hydrangea should never be planted next to concrete foundations or sidewalks. Also, plants in shade need far less fertilizer than plants in the sun.
This article I am posting, has a fairly good list of plants that grow well with hydrangea. Companion planting to me means 'birds of a feather flock together...well'? A landscape design that is great is also inherently low low maintenance. A landscape that is high maintenance is simply a very bad design. Form follows function, not the other way around. Another goal is to make sure that there would be no soil or mulch exposed to the light after 2 years. Major weed reduction! No light no germination of weed seeds...or very little. Healthy plants will have few insect problems or diseases. Weakened plants or plants losing in some competition with a plant more suited to that environment...will be susceptible to insects and disease.
I've never found any basis to companion planting in terms of someone's opinion. Such as marigolds protecting other plants from whatever. You can be sure that plants communicate via chemistry and are very happy to be with plants with the same needs. Healthy garden, healthy soil equals insect and disease resistance.